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

closes #7043
Ah, ok, I think I understand my confusion about `\nFoo`: the `take_next` variable really means "take current", i.e. whether to include the current codepoint `ch` in the grapheme or not. Could you rename it to `take_current`? (Similarly the `next` in `for (next, ch) in ... { idx = next; ...` is actually the offset of the current codepoint `ch`, not the next one, could you adjust that to `curr` or `current` or whatever too?)