refactor: split caches, make API key-focused

Sorry for this breaking change, but I wanted to get the foundations
right before tagging 1.0.

This completely restructures the core of citar to borrow some code and
ideas from the org-mode oc-basic package.

In particular, it changes to using two primary caches:

- bibliography
- completion

Both of these now use hash tables, rather than lists.

Caching functionality is also changed, and the API now focuses on
citekeys as arguments for key functions.