Attempting to format a document when rustfmt isn't installed will result
in an error being returned to the frontend. An alternative
implementation would be returning zero replacements.
Note that VSCode asks for completion after *first* `:` as well:
use crate:
we use hacks to protect against that, and to give completions only
after the second `:`.
Implements a pretty barebones function signature help mechanism in
the language server.
Users can use `Analysis::resolve_callback()` to get basic information
about a call site.
Fixes#102
