Ocaml-protoc

This library is two things in one:

• stubs for the runtime component (now in the "pbrt" package, see Pbrt)
• some internal compiler library, as Ocaml_protoc_compiler_lib.

For most users, the normal workflow is to use the ocaml-protoc compiler executable to generate code, and link the generated code with the pbrt library. No runtime dependency on "ocaml-protoc" should be needed.