Cache rustybuzz shape plans for vastly improved performance.

[vorporeal]

I added support to rustybuzz to cache shape plans (harfbuzz/rustybuzz#88), which has been released in rustybuzz v0.12.0.

This PR updates cosmic-text to make use of the new rustybuzz functionality by adding a ShapePlanCache, storing shape plans (with an appropriate cache key), and passing the cached shape plan to rustybuzz::shape_with_plan (instead of rustybuzz::shape) to avoid re-paying the cost of computing the shape plan.

A couple notes:

• I renamed cache.rs to glyph_cache.rs now that there's a shape_plan_cache.rs, but kept the names of symbols within it generic (e.g.: Cache) as they're public outside of the crate.
• The shape plan cache does not have a bounded size. I believe this is ok, due to the fact that:
  1. Nothing sets a language on the shaping buffer,
  2. I would expect there to only be one direction per script value (not to mention the fact that the max multiplication factor is 2), and
  3. I would expect there to usually be only one script value per font (e.g.: Latin/Hebrew/Chinese).
• Exposing the ShapePlanCache via FontFallbackIter is nasty, but there isn't a great alternative given the fact that ShapePlanCache is held by FontSystem and FontFallbackIter holds a &mut FontSystem and we need a &mut ShapePlanCache in shape_fallback. That said, this nastiness is narrowly scoped and entirely internal to this crate.

All tests in cosmic-text still pass with this change.

For some anecdotal evidence on performance improvements - all font shaping/layout performance issues we were seeing in our app (https://github.com/warpdotdev/Warp) have disappeared with this patch.

[jackpot51]

Thanks!

[jackpot51]

If cosmic-text is being used by warp please consider mentioning this in #177