consolidated ctx typography state

- the stylesheet is now part of the _typography tuple
- note that it needs to be cloned in the StyleMixin since the DEFAULT
sheet gets mutated
- (which is to say there’s probably a cleaner way of inheriting the ctx
state without side-channeling the stylesheet)
refactored fontspec parsing & aat features

- added an ordinals option to `vpos`
- calling Font(None) now returns the default helvetica neue/24 face
fixed a caching bug
added opentype support