Merge pull request #76 from darinmorrison/master

Replace local_data_key! with thread_local!; fix #75
small optimisation
Better 'Show' for racer::Path for debugging
Optimisation: delay processing use-globs in a scope until last

 - use globs like 'pub use self::IoErrorKind::*;' are really expensive
   to process since every search needs to follow them if they are in scope.

   Instead delay following them in the hope that an exactmatch will show up
   before we have to.
Fix warnings wrt unnecessary mut