- defers a lot of validation to after the AST is fully constructed
- in particular, defers tag validation to after message options are parsed
  in order to know if it has message_set_wire_format option, which impacts
  allowed tag range
- fixes issues with very large constant numbers (that overflow uint64 or
  underflow int64)
- refactors grammar around option names and fixes issue where extensions
  on message options aren't parsed correctly
- fixes issues related to groups in oneofs
- fixes issues with reserved name validation
- adds some util methods to shrink the boiler-plate for error creation
  and error handling
- breaks up monolithic parser.go into three files: parser.go, validation.go
  and descriptor_protos.go
- adds numerous new validation test cases to catch various issues that
  were fixed