Add ability to pass in custom function to KCFGExplore for modifying new nodes

[nwatson22]

This PR adds the ability to pass in a function simplify_node: CTerm->CTerm to KCFGExplore() which modifies new nodes as they are added during exploration. This is intended to be used to accomplish runtimeverification/evm-semantics#1909.

[ehildenb]

Let's call it abstract_node instead of simplify_node, and make it an argument to APRProver instead of KCFGExplore.

We have have another block like _check_subsume or _check_terminal, which is _check_abstract. https://github.com/runtimeverification/pyk/blob/master/src/pyk/proof/reachability.py#L316. It should:

• See if auto_abstraction exists, if it does, try applying it.
• If applying it changes the cterm, then insert the new cterm into the KCFG, and also insert a cover from the old node to the new one. Then skip the rest of advance_proof (return true from _check_abstract so that it can continue just like the other _check* methods).
• If auto_bastraction doesn't exist, or doesn't result in a change in the cterm, continue with checking extract_branches.
• Make sure to thread through the new argument to the APRBMCProver too.