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