Adds pushLimit: and popLimit: into GPBCodedInputStream

Work for #1866 

Migrates all the public class docs over to appledoc format.  While Xcode is fine with blank lines in `///` comments, appledoc (used by cocoadocs) isn't and was leaving a bunch of info off the doc pages.

The generator still needs to be updated to do this also; that will be a follow up CL.

Work for #1866 

Migrates all the public class docs over to appledoc format.  While Xcode is fine with blank lines in `///` comments, appledoc (used by cocoadocs) isn't and was leaving a bunch of info off the doc pages.

The generator still needs to be updated to do this also; that will be a follow up CL.

Add more context to GPBCodedInputStream failures.
Have GPBMessage parsing apis extract out the GPBCodedInputStream information and expose it.
Update HeaderDocs with pointers to all error domains/codes.
Expand the unittests to cover the full set of errors reported.

Fixes https://github.com/google/protobuf/issues/1618

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.

- Convert most of the core library headers over to HeaderDoc format.
- Switch the generated comments over to HeaderDoc.
- Create GPBCodedOutputStream_PackagePrivate and move some things into there
  that should be more internal.

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.

Special values for float and double make it inaccurate to test the equality with ==.
The main Java library uses the standard Object.equals() implementation for all fields,
which for floating point fields means Float.equals() or Double.equals(). They define
equality as bitwise equality, with all NaN representations normalized to the same bit
sequence (and therefore equal to each other). This test checks that the nano
implementation complies with Object.equals(), so NaN == NaN and +0.0 != -0.0.

Change-Id: I97bb4a3687223d8a212c70cd736436b9dd80c1d7

Change-Id: Ibbe944fff83e44a8f2206e18ee9ec6f10661297a

If has is set, also always serialize.

Change-Id: I2c8450f7ab9e837d722123dd1042991c0258ede3

Like micro protobufs except:

- No setter/getter/hazzer functions.
- Has state is not available. Outputs all fields != their default.
- CodedInputStream can only take byte[] (not InputStream).
- Repeated fields are in arrays, not ArrayList or Vector.
- Unset messages/groups are null, not "defaultInstance()".
- Required fields are always serialized.

To use:

- Link libprotobuf-java-2.3.0-nano runtime.
- Use LOCAL_PROTOC_OPTIMIZE_TYPE := nano

Change-Id: I7429015b3c5f7f38b7be01eb2d4927f7a9999c80

Like micro protobufs except:

- No setter/getter/hazzer functions.
- Has state is not available. Outputs all fields != their default.
- CodedInputStream can only take byte[] (not InputStream).
- Repeated fields are in arrays, not ArrayList or Vector.
- Unset messages/groups are null, not "defaultInstance()".
- Required fields are always serialized.

To use:

- Link libprotobuf-java-2.3.0-nano runtime.
- Use LOCAL_PROTOC_OPTIMIZE_TYPE := nano

Change-Id: I7429015b3c5f7f38b7be01eb2d4927f7a9999c80