Choice of font fallback API on macOS

Nobody has claimed this yet.

My writing on font fallback (see [font fallback blog post](https://raphlinus.github.io/rust/skribo/text/2019/04/04/font-fallback.html)) has generally suggested using [CTFontCopyDefaultCascadeListForLanguages](https://developer.apple.com/documentation/coretext/1509992-ctfontcopydefaultcascadelistforl). Responding on [Twitter](https://twitter.com/jjgod/status/1124697964058832898), Jiang Jiang suggests we use [CTFontCreateForString](https://developer.apple.com/documentation/coretext/1509506-ctfontcreateforstring) instead.

This is much more like the Windows approach, and saves the trouble of having to go through a long font list, but on the other hand involves calling into CoreText API's on a fine grained basis every time a layout is needed. Part of my idea of getting the cascade list is that it could be retained on a per-locale basis.

Probably measurement is needed to figure out which is better. If CTFontCreateForString is very fast, then maybe that is the best choice.

Compare CTFontCopyDefaultCascadeListForLanguages with CTFontCreateForString for macOS font fallback, including the per-locale cascade-list approach and fine-grained CoreText calls described in the issue. Measure their behavior during text layout and use the results to recommend which API approach Skribo should adopt.

Written by the indexing model from the issue text.