rename std::iterator to std::iter

The trait will keep the `Iterator` naming, but a more concise module
name makes using the free functions less verbose. The module will define
iterables in addition to iterators, as it deals with iteration in
general.

src/libstd/to_bytes.rs

@@ -18,7 +18,7 @@ use cast;
 use container::Container;
 use io;
 use io::Writer;
-use iterator::Iterator;
+use iter::Iterator;
 use option::{None, Option, Some};
 use str::{Str, StrSlice};
 use vec::{Vector, ImmutableVector};