This is in preparation for making upb_def capable
of parsing binary descriptors directly.
We leave upb/descriptor/descriptor.upbdefs.* in
place for now, because upb/descriptor/reader.*
still depends on it.
Also removed a bit of cruft from the codegen.
Also includes an implementation of the conformance tests
to display what the API usage will be like.
There is still a lot to do, and things that are broken (oneofs,
repeated fields, etc), but it's a good start.
Josh Haberman