better reuse of lookups for inline contextual ligature substitutions

[Colin Rofls]

Currently we will create a new lookup for each inline contextual ligature substitution; that is, given the fea,

feature test {
    sub f' i' by f_i;
    sub f' f' by f_f;
} test;

we will generate three lookups: one contextual lookup and two ligature substitution lookups.

I'm not sure how commonly used this syntax is, and our behaviour does match fonttools, but it doesn't match afdko (which combines the lookups).

So low priority, but another situation where some optimization is possible.