Handle Any formatting for diagnostic purposes

Make nuget package support coreCLR

Add JSON parsing to conformance tests.

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

The sources themselves appear to already be Python 3 clean.

Fix headers for gmock.BUILD