[GSUB/GPOS] Add get_coverage() and use it to speed up main loop

[GSUB/GPOS] Add get_coverage() and use it to speed up main loop

And use it to speed up the hotspot by checking coverage directly in
the main loop, not 10 functions deep in.

Gives me a solid 20% boost with Indic test suite.  Less so for less
lookup-intensive scenarios.

Remove the "fast_path" hack from before.