Add consumers and formalize specification

[pineapplemachine]

Add consumers and formalize specification

[pineapplemachine]

Consumer specification, after lots of rumination. These are attributes of the object to be passed to the defineConsumer function.

summary - same as sequence types
docs - same as sequence types
tests - same as sequence types
converter - same as sequence converter attribute, but applies to consumers instead
constructor - same as sequence types
pushFront - push an element from the front of a sequence
pushBack - push an element from the back of a sequence (optional!)
pushEnd - inform the consumer that there are no more elements to push
done - return true when the consumer either matched or didn't match the subject
match - return true when the consumer matches the subject
copy - return a copy of this consumer

The distinction between done and match is slightly weird:

done is true when the consumer won't accept more elements. It means either, "I've matched a complete pattern and if you push any more I will stop matching it", or "I've failed to match the pattern and nothing you push now will change it"

match is true when the consumer matches the subject. It does not necessarily imply that, after pushing another element, the consumer will no longer match it. To get the full (longest) match of a consumer, one should keep pushing until done is true and use the most recent element where match was once true to be the end of the match. (If match was never true, then there is no match at all.)

[pineapplemachine]

Consumers should also support reversing where possible