parser feature-parity with Google protoc

[ctaggart]

• Load and parse files from import statements.
  See https://developers.google.com/protocol-buffers/docs/proto3#importing-definitions
• Verify message identifiers used as field types are actually defined
  and enum values used in option value assignment are actually
  defined. Note these must handle forward references.
• For option assignment, verify type of literal matches type of option's
  message field definition.
• Predefine standard options and enums, or load these from an
  external "descriptor.proto" file. For standard option definitions,
  see https://github.com/google/protobuf/blob/master/src/google/protobuf/descriptor.proto#L251
• Generate errors with context including file/line/column when
  any of the above validations fail (note: this might be better than
  what protoc reports).
• Record enough source information in the AST to generate SourceCodeInfo
  (see https://github.com/google/protobuf/blob/master/src/google/protobuf/descriptor.proto#L84)
• Record any other information in the AST needed to generate a complete
  FileDescriptorProto and FileDescriptorSet.

[haf]

Ref grpc/grpc#12471 – would be nice to avoid C# code generation in the F# example.