Split out codegen into crates

[arlyon]

I have been doing some planning on how we can rework the api for 0.15. This will be a minimal outcome for end users but will change the way types are managed and reexported internally to enable parallel compilation.

In 0.14 I split the codegen to work in terms of Files. A file logs all of its dependencies and these are added when the file is written. The goal is to

• expand the import tracking metadata to track the crate it is coming from - initially the crate will just be stripe
• define some metadata on organising crates
• write a crate generating module that will create a cargo file, generate all the dependencies, and add a mod that exports all the items inside it
• resolve dependencies between cargo files (and crates)