Update codegen

[MendyBerger]

Resolves #218

Implemented according to what was agreed upon in the following comments:

• Overall design
• Version
• Decorators

Since this is quite a lot of code, I've split it into 3 commits:

• Codegen code changes
• Existing tests updates
• New tests

[dicej]

Thanks for doing this (and for the excellent test coverage), @MendyBerger!

Looks good to me; my only concern is the remove_dir_all added to BindingsGenerator::generate, which could be a surprising change from the previous behavior. I'd prefer to avoid that and instead simply refuse to generate the bindings by default if the directory already exists (in which case the user can remove it themself), with an option to force the existing directory to be used if desired.