Clean up `CodeGenerator` by moving compilation-global data and logic to `Context`

[mzabaluev]

Refactoring improvements to prost-build:

• Move compilation-global context data and logic around it from CodeGenerator to a new Context struct.
• Lift similar logic into Context from MessageGraph, where it caused some data duplication.
• Resolve some awkwardness about the Config reference needing to be mutable for its user-provided service generator, by exposing both from Context in a more disciplined way.

[mzabaluev]

I would like this in, before I add an enum type registry, which is necessary to properly implement imports (and later with editions, the enum_type feature) for #1079.