Generate stem hints

Generate stem hints

These are generated from the glyph bitmap by a similar
pattern-matching arrangement that generates the outline.  Then, like
emit_path(), there's an emit_hints() that turns them into valid
charstrings.  The handling of overlapping hints could be cleverer: at
the moment we emit the leftmost-possible set of hints, but we collect
a histogram of where hints are needed so could instead emit the most
useful.  Or even try to do hint substitution.

This makes the Bedstead Web page at 90% zoom much prettier than
before, but it's not as good as the FontForge auto-hinter managed.
Edge hints and counter hints will probably help.