Renaming of the Iter types as in RFC #344

libcore: slice::Items -> slice::Iter, slice::MutItems -> slice::IterMut
libcollections: *::Items -> *::Iter, *::MoveItems -> *::IntoIter, *::MutItems -> *::IterMut

This is of course a [breaking-change].

Some Unstable attributes of functions affected might not longer need the attribute since the return type is now correctly named.

libstd::collections::hash is left out since the rename there is handled in a different PR as far as I know.

@Gankro @aturon