* Fix ruby gc_test in ruby 2.4

* Initialize global variables to Qnil.

This makes the frame stack per-parser, and per-thread.  Fixes #3250

This makes the frame stack per-parser, and per-thread.  Fixes #3250

Change-Id: I6cf77f01370204ad4bc7b345a040a9a3de1706a0

While we are C99 in general, the Ruby build system
for building C extensions enables several flags that
throw warnings for C89/C90 variable ordering rules.
To avoid spewing a million warnings (or trying to
specifically override these warnings with command-line
flags, which would be tricky and possibly fragile)
we conform to Ruby's world of C89/C90.

Change-Id: I0e03e62d95068dfdfde112df0fb16a248a2f32a0

Since this version of upb supports C89, all of the
extra compiler flags are no longer required.

This adds the Map container and support for parsing and serializing maps
in the protobuf wire format (as defined by the C++ implementation, with
MapEntry submessages in a repeated field). JSON map
serialization/parsing are not yet supported as these will require some
changes to upb as well.