large refactor, plugins, single-file codegen

[c-cube]

The goal of this PR is to streamline the code generation for ocaml-protoc and cleanup the code. This would help me add new code generation plugins[^1] and also reduce the sprawl of files produced by ocaml-protoc.

This is very much breaking:

• options are now long (--pb and --pp instead of -pb and -pp)
• all code for a .proto now lives in a single .ml/.mli file

In addition, the compiler lib can be directly used to generate code from a .proto, giving it a list of plugins, without requiring Cmdline.t.

[^1]: my plans are: expose mutable records (or have an option to directly make all records mutable in the first place); and codegen for services

[c-cube]

@Lupus this is not finished but it's what I had in mind. With this it should be easier to add more Plugin.t to generate services, etc.