This allows us to pass it a `&mut DocContext` which will allow removal
of RefCells, etc. in the following commits. It's also somewhat a unique
Clean impl in that it previously ignored `self` (re-retriveing
hir::Crate), which it no longer needs to do.

We have &mut access, so remove the RefCell borrowing

The alternate mode merely prints out the passed in text which is largely
useless (as the text can simply be directly printed).

Previously we stored the entire current path which is a bit expensive
and only ever accessed its length. This stores the length directly.

This introduces a WithFormatter abstraction that permits one-time
fmt::Display on an arbitrary closure, created via `display_fn`. This
allows us to prevent allocation while still using functions instead of
structs, which are a bit unwieldy to thread arguments through as they
can't easily call each other (and are generally a bit opaque).

The eventual goal here is likely to move us off of the formatting
infrastructure entirely in favor of something more structured, but this
is a good step to move us in that direction as it makes, for example,
passing a context describing current state to the formatting impl much
easier.