Add syntax highlighting support for GtkSourceView / GEdit
auto merge of #5166 : danniel/rust/incoming, r=brson

I copied and adapted the syntax highlighting rules from Kate, to be used in GEdit and other GtkSourceView based editors.
auto merge of #5113 : alexcrichton/rust/issue-4366, r=catamorphism

The first two commits are the actual fix going into place, and the last is just dealing with the fallout in the rest of the compiler.

The test added in the first two commits had 0 failures before this patch, and if the glob imports were changed to explicit imports then the errors showed up. Due to this behavior, I figured that the desired behavior was for glob imports to not accidentally leak a lot of non-public imports/functions/types into other modules.

There was quite a lot of fallout, and it all passes `make check` locally, but I doubt that it will pass on all platforms because there's probably some guarded off thing I missed.

I plan on making another patch soon which changes the default level of `unused_imports` to `warn` which will hopefully reduce a lot of the `use` noise throughout. In conjunction with #5104, and a few minor fixes, I think that the default level of `warn` is actually really usable.
Change the default level of unused_imports to warn
Remove unused imports throughout src/
Add a special FIXME for subduing warnings in rpath.rs
Adding missing imports for tests