Very much work in progress.

See unicode-rs#21

Implemented next_boundary and prev_boundary functions in terms of
is_boundary (plus fixups to the internal state when moving the
cursor). Fixed various problems in previous commit.

Still work in progress, not tested yet.

Also, additional state machine work, mostly resume logic in next and
prev grapheme cluster boundary methods.

Includes some documentation, and also a bit of renaming from the
earlier development drafts.

This adds a test case for unicode-rs#19 (which was a mismatch between forward
and reverse iterators in the original codebase).

Some of the doc tests also encouraged me to tweak the implementation.

The existing code treated CR and LF as special cases of the Control
grapheme category, for reasons that weren't very good. This patch gets
rid of that and just handles GB3 in the pair lookup. That should
improve performance in the rope case, as it will cut down on the
amount of pre-context requested when a chunk begins with LF.