Replace ttf_parser, rustybuzz with read-fonts, HarfRust

[dhardy]

Closes #111: migrates away from ttf-parser (officially no longer maintained) and rustybuzz (unmaintained) to read-fonts and HarfRust.

This loses support for variations in some font metrics (though not in shaping). Likely this is rarely observable anyway. (It might be when using font variations without the shaping feature. Kerning will also be broken in that case. It still may be better for user experience to just let the library output badly spaced glyphs than error out so I won't add a lint.)

Note: read-fonts is clearly a much lower level and less opinionated library than ttf_parser, and is significantly harder to use as a result. It may be more efficient (e.g. this approach lets us read and store only the kerning sub-tables we will use), though I can't tell whether re-constructing tables on-demand is the intended way to use them vs storing the table with its redundant pointer vs extracting all desired data during the initial read (though fns like Hmtx::advance are clearly not designed for the latter).