add Graphemes iterator; tidy unicode exports

- Graphemes and GraphemeIndices structs implement iterators over
  grapheme clusters analogous to the Chars and CharOffsets for chars in
  a string. Iterator and DoubleEndedIterator are available for both.

- tidied up the exports for libunicode. crate root exports are now moved
  into more appropriate module locations:
  - UnicodeStrSlice, Words, Graphemes, GraphemeIndices are in str module
  - UnicodeChar exported from char instead of crate root
  - canonical_combining_class is exported from str rather than crate root

Since libunicode's exports have changed, programs that previously relied
on the old export locations will need to change their `use` statements
to reflect the new ones. See above for more information on where the new
exports live.

closes #7043
[breaking-change]

src/libstd/io/mod.rs

@@ -237,7 +237,7 @@ use str::{Str, StrSlice};
 use str;
 use string::String;
 use uint;
-use unicode::UnicodeChar;
+use unicode::char::UnicodeChar;
 use vec::Vec;
 
 // Reexports