Add support for character alternates

[ZevEisenberg]

kCharacterAlternativesType is where it's at. All the cool kids are doing it. It's used to enable specific alternate characters in some fonts, like alternate ampersands, vertically-centered colons, and different ligature styles.

It may be difficult to make this one work with Comparable conformance, because there are 99 possible values (1-99), and while a 20-member struct worked OK for stylistic alternates, a 100-member struct may be pushing it. It would work as a fallback, but it bears discussion. We may choose to drop some comparability features.

[ZevEisenberg]

We could also generate a file, instead of writing it by hand.