Rename two 'cvXX' features based on SAA5054 shapes

Rename two 'cvXX' features based on SAA5054 shapes

The features that generate the variant ocircumflex and ccedilla glyphs
are now treated a being variants of lower-case 'o' and cedilla
respectively.  Unlike the SAA5051/2 variants, we don't yet have
variants of their base glyphs.  Indeed, for the lower-case 'o' we
probably never will, because the variant is "larger accented versions"
so the unaccented version wouldn't change.

I haven't done anything about the weird SAA5054 ugrave because I'm not
sure there's any real design principle behind it.

I've also added a comment explaining the numbering of 'cvXX' features.