Warn about unused imports

[catamorphism]

I don't see a bug for this already, but it would be nice to have a warning for unused imports.

[marijnh]

So far, my experience has been that these warnings are mostly very annoying. I don't want to have to re-tune my imports after every change to make sure they precisely cover the things I'm actually using.

Also, import foo::bar; import bar::{a, b}; warns about unused import bar, although it is used by another import.

Reopening for discussion.

[lht]

How about adding a flag like fwarn-unused-imports and turn it off by default?

Regarding the cascaded import warning, it should be a bug. I will try to look at it.

[marijnh]

An off-by-default flag would be an improvement I guess -- one could actively polish up imports now and then, to make sure they aren't misleadingly broad, without having to constantly worry about it.

[lht]

Added the flag in 32868f0.

[lht]

I rebased master to fix a merge conflict. And the incorrect warning bug is fixed also. Please review.

3945548 rustc: Add a flag '--warn-unused-imports'
32cf972 rustc: Fix warn on unused import bug