New lint : unused_extern_crate. #10385
Removing unused extern crates.
auto merge of #17134 : vberger/rust/lint_unused_extern_crate, r=alexcrichton

This PR creates a new lint : ``unused_extern_crate``, which do pretty much the same thing as ``unused_import``, but for ``extern crate`` statements. It is related to feature request #10385.

I adapted the code tracking used imports so that it tracks extern crates usage as well. This was mainly trial and error and while I believe all cases are covered, there might be some code I added that is useless (long compile times didn't give me the opportunity to check this in detail).

Also, I removed some unused ``extern crate`` statements from the libs, that where spotted by this new lint.