investigate changes to harfbuzzjs to reduce opentype processing

[jpt]

I think we can get rid of a few font binary processing things by adding them to harfbuzzjs. in harfbuzz/harfbuzzjs#132, khaled sounded receptive to metrics, if it doesn't inflate the binary by too much. on the one hand, it would make sense to use our own binary, if it isn't too hard to make it expose some of these things, on the other hand, though, I'd prefer not to distribute a binary

[jpt]

in progress, but looks like it could add 8kb or so minimum, need to double check some things. not sure if that will ultimately be acceptable upstream