Partially Revert "Replaced all instances of Simple{IDF}toa with StrCat."

This reverts commit 029f2c7c.

* Add a new conformance test for top level JSON null

Replaced all instances of Simple{IDF}toa with StrCat.

Update the version for JavaLite in update_version script

Down integrate to GitHub

Down Integration to GitHub

This makes for easier rewrites between internal and external.

All bazel crosstools automatically pass -Wall to compiles. The order of
operations is:

- bazel crosstool flags
- `--host_copt` flags and other variations like `--host_cxxopt`
- the `copts` defined on the rule

Because of this when protobuf produces warnings, there's no way to
disable them from the consumer side if they are re-enabled later by
another `-Wall` flag.

These were found attempting to enable -Werror

GCC 8 added a -Wclass-memaccess that is enabled in -Wall that warns when
making direct memory modifications to non-trivial objects. The objects
that are modified here are only "non-trivial" in non-interesting ways
(e.g., removal of copy constructor) and can be safely modified as is
being done.

This simply explicitly casts the pointers before calling memset to
indicate that they should be treated as raw memory.

Down integrate to GitHub

Remove Required.Proto3.JsonInput.EmptyFieldMask.JsonOutput from failure list since it is now passing for Ruby

Replace SimpleItoa with StrCat in ObjectiveC compiler codes

https://groups.google.com/d/msg/protobuf/H8_D1hfg3p4/A2oPHdfUAgAJ

When compiling protobuf these produce warnings

This field isn't used yet, but it will be in the future so we don't want
to remove it. This fixes the warning in the meantime.

Down integration to GitHub

Update documentation for JavaLite

Add Mergeable config