Make memo a queue

[parzhitsky]

memo is implemented as a simple JavaScript array (a stack). When being limited, it should have elements removed from its head, and this operation is O(n) in stacks, which is a pain in the arse. Would memo be implemented as a queue, this operation would be O(1), but the API might be changed in a way that's not intuitive for JavaScript developers.

Make sure that:

• memo is a queue (in both Predicate and NextFactory);
• all elements of memo are properly index-accessible, similar to arrays (Proxy?);
• .unshift() is an alias for .enqueue(); .pop() is an alias for .dequeue();

It is worth noting for anybody who is reading this, that mutating memo is undefined behavior, and could lead to nasty hard-to-trace bugs.