Fix toot length tracking with nontrivial Unicode.

Fix toot length tracking with nontrivial Unicode.

Somehow I totally forgot to distinguish chars from bytes in
Composer::make_regions(), _and_ forgot to test it. So I had a crash
this morning when editing a toot containing Unicode put the 500-byte
mark midway through a UTF-8 character.

That's wrong for _two_ reasons - firstly, no endpoint in the regions
array ought to be anywhere other than a character boundary, and
secondly, the 500 _byte_ boundary has nothing to do with the toot
length limit in the first place! The 500 _character_ boundary would
have been more to the point.