Optimize auto-serif behavior of four characters.

[jmcwilliams403]

Originally this was just going to be for returning auto-serifed behavior to Greek Lower Digamma/Heta to save a handful of glyphs, but I extended the scope to improve variant selection for forced-single-storey a and g to facilitate #2552 so that one of its single-storey g-derived characters doesn't show duplicate variants once its auto-serifed variants are exposed in the specimen.

Showing all a variants as an example:
sans before:

sans after (in particular look at cv36-02/07):

slab before:

slab after (effectively unchanged):

To show g variants in the same way would not show an obvious difference except for in the variant atlas in the release build, so I'll spare you the images for bandwidth reasons.