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
Ok, I checked locally: reverting this change reduces the diffstat to

```
5 files changed, 1335 insertions(+), 58 deletions(-)
 ...
 src/libunicode/tables.rs  | 885 +++++++++++++++++++++++++++++++++++++++++++++-
```

rather than
```
5 files changed, 4104 insertions(+), 2767 deletions(-)
 ...
 src/libunicode/tables.rs  | 6363 ++++++++++++++++++++++++++-------------------
```

as it is now. Please revert it, or at least do it in its own separate commit, to keep things as clean as possible.