Fix sparse master compatible hinting

[josh-hadley]

In autohint.py, hint_compatible_glyphs currently uses a hack to deal with sparse masters. The root cause is that the C implementation (_psautohint.autohintmm) does not support sparse masters. The current workaround works perfectly fine, but it would be nice and probably marginally more efficient if we didn't need to use it.

So ideally, we'd like to remove the hack code and be able to pass sparse master data directly to _psautohint.autohintmm and avoid pre-processing tricks in the Python code.

Rough to-do list:

• [ ] update C code to properly handle sparse masters
• [ ] remove Python hack
• [ ] apply any additional updates to Python code to ensure things work smoothly