Add ProofNodeManager and ProofChecker (#4317)

Add ProofNodeManager and ProofChecker (#4317)

Further work on adding core utilities for ProofNode objects, in support of the new proof infrastructure.

ProofNodeManager is analogous to NodeManager. It is a trusted way of generating only "well-formed" ProofNode pointers (according to a checker).

ProofChecker is analogous to TypeChecker. It is intended to be infrastructure for our internal proof checker.

This PR (and 1 more) is required to get to a place where Haniel and I can collaborate on further development for proofs.