Split generator frontends/backend

[endgame]

Splitting the generator front and backend would allow us (or other people!) to make amazonka-style SDKs for their own requests. Some examples: API Gateways with IAM Authentication, Amazon's own Selling Partner API (SP-API).

A first step to doing this would be to make a library which is a direct and detailed representation of botocore types, and write functions to convert that into shape definitions etc.

See also: #763.