Allow ligatures with one component only
Minor
[subset] More fixing of (Chain)ContextFormat2

This is follow up to 98b60757ab9bb0aaa3c1f136e5a837a1b73d22ed
Looks like this lookup type is a curse to stay with us.  Sigh...
[subset] Actually prune empty subrulesets in (Chain)ContextFormat2
[subset] Fix pruning of empty rulesets in (Chain)ContextFormat1

Similar to 98b60757ab9bb0aaa3c1f136e5a837a1b73d22ed and the
recent commits for Format2.
[subset] Add TODO