Ruby: make sure map parsing frames are GC-rooted.

This involved fixing a few important bugs in the
Ruby implementation -- mostly cases of mixing
upb field types and descriptor types (upb field
types do not distinguish between int/sint/fixed/sfixed
like descriptor types do).

Also added protobuf-specific exceptions so parse
errors can be caught specifically.

Change-Id: Ib49d3db976900b2c6f3455c8b88af52cfb86e036

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

Also fixed lines with > 80 char length.

* make consistent between mri and jruby
* create a #to_h and have it use symbols for keys
* add #to_json and #to_proto helpers on the Google::Protobuf message classes

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.

system. The Ruby module build now uses an amalgamated distribution of
upb, and successfully builds a Ruby gem called 'google-protobuf' with
module 'google/protobuf'.

This adds a Ruby extension in ruby/ that is based on the 'upb' library
(now included as a submodule), and adds support for Ruby code generation
to the protoc compiler.