Change-Id: Ibb3fe6f919cc7ca0df91da5e1697ba33d259e433

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

This was enabled by the recent open-sourcing of JSON
support and MessageDifferencer.

MessageDifferencer allows the conformance suite to expand
because it allows us to write tests for payloads that parse
successfully.  To verify the testee's output payload, we
need to parse it back into a message and compare the message
instances.  Comparing output bytes vs. a golden message is
*not* valid, because protobufs do not have a canonical
encoding (especially in the presence of maps, which have
no prescribed serialization order).

We only add one small JSON test for now, but with the
framework in place we now have the foundation to dramatically
expand the coverage of the conformance test suite.

Also added the ability for the testee to skip tests that
exercise features that are unimplemented.  This allows
Java (which currently has no JSON support) to skip tests
involving JSON.

Change-Id: I697b4363da432b61ae3b638b4287c4cda1af4deb

Change-Id: I382dcda97fa123a6da4ff5faad5d7ece95853f33

This allows us to enable conformance tests even when we know
that some tests are failing and need to be fixed.

Change-Id: I372f43663008747db6f2b2cf06e6ffa4c6d85b2d

This is necessary for testing on iOS.

Change-Id: I54ec1e3aa2e9fbfff9a6cd6580920a6a62069b63