Tracking issue: refactor CodegenExtension

[doug-q]

The current interface codegen extensions has a few problems:

• Only one codegen extension per HUGR extension. This is an annoying and unnecessary limitation
• It has an unnecessary lifetime paramater `'c``. Everything should work with a context of arbitrary lifetime.
• Handlers for custom types depend on the H parameter, when they have no reason to.

I suggest changing to a builder pattern, where adding a codegen extension amounts to passing callbacks for ops, types, constants that the extension handlers, finishing by providing glorified HashMaps for each of op, type, constants.

TODO sketch example

• Remove 'c lifetime parameter to CodegenExtension #111
• Remove trait EmitOp #112
• Replace trait CodegenExtension with a builder #115
• Add TypeMap , use it to compute llvm types, put it in TypeConverter. Move the iw_context into TypingSession, remove H generic parameter from TypingSession. #116
• Move extensions to crate::extensions from crate::custom. #121
• Allow extension callbacks to have non-'static lifetimes #127