Create Type.Api

This is the general behaviour of something which can supply a types:

following functionality is expected:

- fetch_spec -> gets a typespec, as defined in a module
- fetch_type -> gets a type, as defined in a module
- ...? of?

Should be generalizable to several different concepts:
- An "override" which lets you define how a type works.
- A "cache system" which lets you store as PLTs or in ets tables.
- An inferencing system which actually looks at the machine code to find the types.

The issue names no files, tests, or entry points. First map the existing type-related modules and their public interfaces, then determine how Type.Api should support fetch_spec and fetch_type across overrides, caches, and inference. Done means the API design and its supported concepts are defined clearly enough to implement and verify.

Written by the indexing model from the issue text.