Drop all use of OSSpinLock

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 region tags to the Go protobuf examples.

Handle Any formatting for diagnostic purposes

Make nuget package support coreCLR

Add JSON parsing to conformance tests.

This will allow us to like to specific snippets of code in the
documentation.  I plan to create a tutorial similar to the C# tutorial
https://developers.google.com/protocol-buffers/docs/csharptutorial

Since that tutorial has sections for populating a proto, parsing, and
serializing, I made a region for each of these for Go.  To make the
populating sample more self-contained, I refactor the listing example
slightly.

This addresses issue #1008, by creating a JsonFormatter which is private and only different
to JsonFormatter.Default in terms of reference equality.

Other plausible designs:

- The same, but expose the diagnostic-only formatter
- Add something to settings to say "I don't have a type registry at all"
- Change the behaviour of JsonFormatter.Default (bad idea IMO, as we really *don't* want the result of this used as regular JSON to be parsed)

Note that just trying to find a separate fix to issue #933 and using that to override Any.ToString() differently wouldn't work for messages that *contain* an Any.

Generated code changes follow in the next commit.

MSVC MP flag to build with multiple processes in visual studio.

Update the min toolchain for iOS/OS X to be Xcode 7

- Let Xcode update the projects, schemes, and info.plists.
- Add workaround for shallow analyzer issues in current Xcode versions (deep analyze gets things correct).
- Tweak the Swift based tests to avoid warnings from Xcode 7's XCTest using optionals for autoenclosure results.
- No longer tag the ObjC iOS travis test as flaky, xctool seems to manage the simulator pretty well.

Fix distclean and maintainer-clean to get more generated files.

Enable fast cpp protos in the python interface.

Arena type traits standardization.

Fix narrowing implicit cast in generated message [NFC]

Fixes #1029

Use angle bracket for well known type proto header.

Use compiler provided static assert and avoid tripping unused-local-typedef warnings.

Fixes #1029

Added util/python/BUILD to provide //util/python:python_headers dependency for building use_fast_cpp_protos

Enable CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION for the projects.

This is adapted from the branch of @xfxyjwf at:

https://github.com/xfxyjwf/protobuf/commit/494716a682ef854168e92231a3cdcc89d587d9b9

and should solve the protobuf compilation problem against nvcc.

Tested against nvcc 6.5 and 7.0.

Making _genproto rules public.

Conformance test implementation for Python.

This enables other xx_proto_library targets to depend on xx_proto_library
targets in different packages, and specifically on xx_wkt_protos.

Add srcs_version = "PY2AND3" in BUILD files