Kill some warnings: unused imports and old #[deny(..)]s.
libcore: implement `str::each_char_reverse` correctly.

Previously, `each_char_reverse` was not iterating at all, so the
testcase never ran properly.
auto merge of #5525 : dbaupp/rust/minor_fixups, r=graydon

Kills some warnings, and implements str::each_char_reverse so that it actually iterates. The test case wasn't detecting a failure, since the loop body was never executed.