(proof-new) Add proof managers and eager gens to arith (#5159)

(proof-new) Add proof managers and eager gens to arith (#5159)

This commit threads ProofNodeManager, EagerProofGenerator, etc
throughout the arith theory into the appropriate locations. These
objects are currently unused, but will be used in future commits.

Crediting Andy, since he set up some of this.

Co-authored-by: Andrew Reynolds andrew.j.reynolds@gmail.com