Merge #1055

1055: Allow keeping list of last matches to check against r=CohenArthur a=CohenArthur

When trying to figure out if a match can follow another, we must figure
out whether or not that match is in the follow-set of the other. If that
match is zeroable (i.e a repetition using the * or ? kleene operators),
then we must be able to check the match after them: should our current
match not be present, the match after must be part of the follow-set.
This commits allows us to performs such checks properly and to "look
past" zeroable matches. This is not done with any lookahead, simply by
keeping a list of pointers to possible previous matches and checking all
of them for ambiguities.

Addresses #947
Closes #947

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>