UnicodeStrSlice: add width() and graphemes() methods

- `width()` computes the displayed width of a string, ignoring the width of control characters.
    - arguably we might do *something* else for control characters, but the question is, what?
    - users who want to do something else can iterate over chars()

- `graphemes()` returns a `Graphemes` struct, which implements an iterator over the grapheme clusters of a &str.
    - fully compliant with [UAX#29](http://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries)
    - passes all [Unicode-supplied tests](http://www.unicode.org/reports/tr41/tr41-15.html#Tests29)

- added code to generate additionial categories in `unicode.py`
    - `Cn` aka `Not_Assigned`
    - categories necessary for grapheme cluster breaking

- tidied up the exports from libunicode
  - all exports are exposed through a module rather than directly at crate root.
  - std::prelude imports UnicodeChar and UnicodeStrSlice from std::char and std::str rather than directly from libunicode

closes #7043
Ah, maybe you could add some more comments, or even use a new enum?