constant_fold_expr() replaces my custom logic for string/integer
literals and final references. It even supports folding constant
expressions which means even {1 + 2} could be precomputed!

This means final references to bool, float, bytes*, and complex values
are no longer supported, but that's okay for now. In a later patch, we
can add proper support for (some of) these types to
constant_fold_expr().

*Also bytes are just in general really annoying to work with (because
the true bytes value isn't stored anywhere, you have to convert the
string representation to bytes instead... ugh) so I don't really care
about final refs to bytes values/literals right now. Byte literals IN
the sets are still supported.

I got tired of trying to compile mypy locally (15 minutes and it's still
not done), let's see what CI has to say.