Generate appropriate schema doc comments from .proto files

- Add parse handling of .proto struct and field doc comments (enums and
their values were already handled)
- Add FBS generation handling of doc comments for structs, their
fields, enums, and their values (requires linking idl_gen_general in
the test binary build)
- Tested using test.proto|golden with doc comments added.  Xcode run of
flattest passes.

Change-Id: Idff64dd8064afba227174ab77d2c7be22d006628