Standardize handling of glyph names for U. F. O. output.

This adds a standards-compliant name mangler and a hash table look-up for glyph names (so that we can iterate names that collide after mangling).

I'm against making gnulib a submodule of FontForge. Bootstrap already clones gnulib if necessary, plus it limits the clone depth to 365 - it's not like I need the whole gnulib history.

Additionally, if you look at the bootstrap configuration, a user can specify their own gnulib installation with `--gnulib-srcdir`, so making gnulib a submodule doesn't make sense to me.