Make a "Virtual Doc" for things like keyhints

[e-matteson]

For the future: make a "Virtual Doc" that implements PrettyDocument, b.c. ASTs have a terrible API for this use case, as demonstrated in this method's code. (Add a TODO.)

Originally posted by @justinpombrio in #52

[e-matteson]

After this is done, consider sticking a reference to the keymap manager in the FilteredKeymap struct, and moving the lookup etc. methods onto it. Currently the update_key_hints(&mut self) method makes that impractical for borrow checking reasons.