Added codex-visit-element.

M-, to pop back to previous loc forthcoming. I also need to look into why M-.
is doing weird shit on Mac OS. Yay for crazy keyboards.
A bunch of tidying and revampery.

- All the Codex stuff is consolidated into ProjectCodex.

- Source indexing is farmed out in a way that allows us to do in on a
  background thread.

- We request a reindex and revisit every time a source file is saved. We really
  need to start consolidating our compilers. Pretty soon we'll have a project
  recompile on save, an analyzer re-run on save, and a Codex reindex on save.
  Party time!