(proof-new) Use bound variable manager (#5679)

(proof-new) Use bound variable manager (#5679)

This uses BoundVarManager for several key places where bound variables are introduced, including for array extensionality witness terms, string preprocessing, quantifiers rewriting, and skolemization.

This is motivated by making certain steps in solving deterministic for the sake of proofs, and gives a more consistent pattern in general for constructing bound variables.