Add cvXX (character variant) lookups

Add cvXX (character variant) lookups

The idea of these is that each one allows for choosing between different
variants of a single character (or set of closely-related characters).
This allows for deciding that you want (for instance) the angular
SAA5054 "ccedilla", but not its weird conjoined "ugrave".

This seems like a good way to handle chips like the SN74S262 where some
samples seem to vary in a single character from others, as well as more
generally for making the variant glyphs easily accessible for purposes
other than precise emulation.

At the moment, each feature only provides alternatives for a single
character, but now that we have a mechanism for it, that might change.