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.
Remove unused imports throughout
syntax: Removing uses of HashMap
doc: Remove mentions of oldmap::HashMap
rustc: Remove uses of oldmap::HashMap
rustc: Purge of HashMap, fixing segfaulting cases

Various XXX comments added around to denote copies which when removed cause the
compiler to segfault at some point before stage2. None of these copies should
even be necessary.
test: Remove uses of oldmap::HashMap
Move ast_map::map to LinearMap
std: Remove the oldmap module