Consider an AST cache

[Infernio]

Use pickle (or something else) to serialize the AST and cache it. Then we can skip parsing on repeat commands.

Consider a session like this:

$ zoia build
$ # Build fails, so I fix the errors
$ zoia build
$ # Success! Now to fix my awful formatting
$ zoia fmt
$ zoia check
$ # Uh oh, a bunch of grammar/typo warnings
$ # I fix them, but realize that I repeated one mistake in a few other places
$ zoia grep -i "rust-grown"
$ # Fix those, and:
$ zoia check
$ # All good!

In the process of this session, we parsed the entire project, from scratch, six times. Even though only a handful of files changed at most.