Integrate incremental hom search with rewriting programs

[kris-brown]

Each of the boxes of a graphical rewriting recipe has various hom sets it needs with respect to the current state of the world. Currently these are generated from scratch whenever they are needed (for example, consider a Query block that needs to know all the paths of length 2 in the current graph).

There are certainly scenarios where it would be better to not maintain the hom sets incrementally. For example, if we pass through that Query block just once and never again, then it's pointless to maintain the hom set through every change we make in a possibly very long simulation. So some decision should be made on how to opt-in or opt-out (perhaps on a box-by-box basis) to incremental hom search.

Furthermore, because rewriting programs presently only work for Mark As Deleted ACSets, the incremental hom search will have be to made to support that ACSet type too.