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]

0 files changed, 0 insertions, 0 deletions