Support designspace-style "rules" for location-based glyph substitution

[justvanrossum]

This is from a user request.

See:

• https://fonttools.readthedocs.io/en/stable/designspaceLib/xml.html#rules-element

This is somewhat tangential to proper GSUB support, similar to how support for kerning would not require full GPOS support. (To be clear: not that we don't want those things, but.)