- Don't prune the extension registry as that can lead to failures when two
  threads are racing.
- If adding the method fails, check and see if it already is bound to decide
  the return result. Deals with threading races binding the methods.

GPBMessageDropUnknownFieldsRecursively() is a new helper to drop the
unknownFields from a message and all sub messages (in fields or extensions).

- If setting/clearing a repeated field/map that was objects, check the class
  before checking the autocreator.
- Just to be paranoid, don’t mutate within copy/mutableCopy for the autocreated
  classes to ensure there is less chance of issues if someone does something
  really crazy threading wise.
- Some more tests for the internal AutocreatedArray/AutocreatedDictionary
  classes to ensure things are working as expected.
- Add Xcode 8.2 to the full_mac_build.sh supported list.

- Capture the version used to generated.
- Check at compile time and runtime that generated code isn't from a newer
  version, also check that the min version required is also supported.
- Keep the old constants/macros/functions to special case the last version
  that was working so those generated sources still work until we decide
  otherwise.

If a message is proto3, then the zero values still count as being set one the
field is in a oneof.

Add tests to confirm oneofs work as expected in both syntaxes.

Fixes https://github.com/google/protobuf/issues/1933

Add a new test that forces strings into two different implementations from the
NSString class cluster to help confirm we're exercising both paths by which
CodedOutputStream will extract data from an NSString.

Move the old +load test (that was flawed because the behavior really depends on
the type of string from the NSString class cluster); into a unittest that
targets the specific case we're adding a behavior confirmation on.

As a bonus, improve the TextFormat generation of string characters < 0x20.

If a message is proto3, then the zero values still count as being set one the
field is in a oneof.

Add tests to confirm oneofs work as expected in both syntaxes.

Fixes https://github.com/google/protobuf/issues/1933

Add a new test that forces strings into two different implementations from the
NSString class cluster to help confirm we're exercising both paths by which
CodedOutputStream will extract data from an NSString.

Move the old +load test (that was flawed because the behavior really depends on
the type of string from the NSString class cluster); into a unittest that
targets the specific case we're adding a behavior confirmation on.

As a bonus, improve the TextFormat generation of string characters < 0x20.

Exposes the currently registered extensions for a message and removes the internal sortedExtensionsInUse

- Correct impl by using helpers the message wiring does.
- Add unittests.

Fixes https://github.com/google/protobuf/issues/1716

This is currently checking for the wrong class for enums
and NSMutableArray fields.

Working on https://github.com/google/protobuf/issues/1599, specifically:
- Turn on more warnings that the Xcode UI calls out with individual controls.
- Manually add:
  -Wundef
  -Wswitch-enum
- Manually add and then diable in the unittests because of XCTest's headers:
  -Wreserved-id-macro
  -Wdocumentation-unknown-command
- Manually add -Wdirect-ivar-access, but disable it for the unittests and in
  the library code (via #pragmas to suppress it). This is done so proto users
  can enable the warning.

This will lower the amount of dispatch_semaphores created per Message when the
full object tree isn't walked in a way that would require them to be created.
Uses a dispatch_once_t for one time init of the dispatch_semaphore.

NOTE: This is a binary breaking change as structure sizes have changed size
and/or order.

- Drop capturing field options, no other options were captured and other mobile
  targeted languages don't try to capture this sort information (saved 8
  bytes for every field defined (in static data and again in field descriptor
  instance size data).
- No longer generate/compile in the messages/enums in descriptor.proto. If
  developers need it, they should generate it and compile it in. Reduced the
  overhead of the core library.
- Compute the number of has_bits actually needs to avoid over reserving.
- Let the boolean single fields store via a has_bit to avoid storage, makes
  the common cases of the instance size smaller.
- Reorder some flags and down size the enums to contain the bits needed.
- Reorder the items in the structures to manually ensure they are are packed
  better (especially when generating 64bit code - 8 bytes for every field,
  16 bytes for every extension, instance sizes 8 bytes also).
- Split off the structure initialization so when the default is zero, the
  generated static storage doesn't need to reserve the space. This is batched
  at the message level, so all the fields for the message have to have zero
  defaults to get the saves. By definition all proto3 syntax  files fall into
  this case but it also saves space for the proto2 that use the standard
  defaults. (saves 8 bytes of static data for every field that had a zero
  default)
- Don't track the enums defined by a message. Nothing in the runtime needs it
  and it was just generation and runtime overhead. (saves 8 bytes per enum)
- Ensure EnumDescriptors are started up threadsafe in all cases.
- Split some of the Descriptor initialization into multiple methods so the
  generated code isn't padded with lots of zero/nil args.
- Change how oneof info is feed to the runtime enabling us to generate less
  static data (8 bytes saved per oneof for 64bit).
- Change how enum value informat is capture to pack the data and only decode
  it if it ends up being needed. Avoids padding issues causing bloat of 64bit,
  and removes the needs for extra pointers in addition to the data (just the
  data and one pointer now).

Apple engineers have pointed out that OSSpinLocks are vulnerable to live locking
on iOS in cases of priority inversion:
. http://mjtsai.com/blog/2015/12/16/osspinlock-is-unsafe/
. https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html

- Use a dispatch_semaphore_t within the extension registry.
- Use a dispatch_semaphore_t for protecting autocreation within messages.
- Drop the custom/internal GPBString class since we don't have really good
  numbers to judge the locking replacements and it isn't required. We can
  always bring it back with real data in the future.

Add the clang annotations to the objc library and generated code to help with Swift bridging and compiler checks.

- Add more to the ObjC dir readme.
- Merge the ExtensionField and ExtensionDescriptor to reduce overhead.
- Fix an initialization race.
- Clean up the Xcode schemes.
- Remove the class/enum filter.
- Remove some forced inline that were bloating things without proof of performance wins.
- Rename some internal types to avoid conflicts with the well know types protos.
- Drop the use of ApplyFunctions to the compiler/optimizer can do what it wants.
- Better document some possible future improvements.
- Add missing support for parsing repeated primitive fields in packed or unpacked forms.
- Improve -hash.
- Add *Count for repeated and map<> fields to avoid auto create when checking for them being set.

- Style fixups in the code.
- map<> serialization fixes and more tests.
- Autocreation of map<> fields (to match repeated fields).
- @@protoc_insertion_point(global_scope|imports).
- Fixup proto2 syntax extension support.
- Move all startup code to +initialize so it happen on class usage and not app startup.
- Have generated headers use forward declarations and move imports into generated code, reduces what is need at compile time to speed up compiled and avoid pointless rippling of rebuilds.