subset: speed up subsetting of large fonts
subset: speed up subsetting of large fonts

Two small changes that significantly speed up subsetting of large fonts
such as Noto Sans CJK:

1. When emptying a charstring, simply empty its program rather than
attempting to decompile it first. (Only relevant when retaining GIDs.)

2. When reindexing charstrings, swap an accidentally-quadratic
implementation for one that is linear in the number of retained glyphs.

googlefonts/fonttools,fonttools/fonttools