* Include span label in rust lints
This turns relatively unhelpful error messages like
mismatched types
into more expressive messages along the lines of
mismatched types: expected bool, found integral variable
Fixes#597.
* Exclude rust lint span label if empty
* Use single-quoted strings in vimscript
* Add test for detailed rust errors
* Prune Cargo JSON
* Use matching error file name
* Byte offsets not char offsets
Jon Gjengset