Add ExtensionSet tests and add checks that non-message extension fields don't fail initialization checks

Adjust APIs for extensions to properly return default values for extensions
Fix issues with IsInitialized and proto2 field reflection

Fix generated group code missing end group tag case
Fix generated field classes searching for presence index for extensions

Removed ByteCountInt64.  This is a transitional typedef that should n…

* Test singular fields are encoded in canonical way

* Defautl values in proto3 should not be encoded.
* Values should be converted to the canonical representation (e.g.,
long int64 value may be truncated for int32 field)

* Update failure list

* Update failure list

clang-cl requires the order to be the opposite of what it currently is.
It should not affect other compilers since this is the only compiler
that has both macros defined.

Down integrate internal changes to GitHub

Update to Truth 1.0

The previous issue of mktime doesn't work on Mac has been fixed.

We were creating a map decoding frame when starting the *map*,
but clearing the GC slot when finishing each *map entry*.  This
means that the decoding frame could be collected in the meantime.

* Add source code URI to the Ruby gemspec file

As a developer, I would like to easily find the source code origin for this gem when browsing through https://rubygems.org/gems/google-protobuf.
In order to solve this, I've followed instructions at https://guides.rubygems.org/specification-reference/#metadata by adding the source_code_uri metadata key pair.

* Use git tag instead of fixed "master" string

* Substitute the string, not the Gem::Version

Proto2 has been tested for invalid cases. Check implementation of
ExpectParseFailureForProto (https://github.com/protocolbuffers/protobuf/blob/master/conformance/binary_json_conformance_suite.cc#L322)

Fix for race in lazy initialization of handlers.

Optimized layout_mark() for Ruby

This fixes https://github.com/protocolbuffers/protobuf/issues/6532.