Purify substitutions in strings proof reconstruction (#7035)

Purify substitutions in strings proof reconstruction (#7035)

This fixes an issue in strings proof reconstruction where sequential substitutions are used over possibly non-atomic terms like (str.replace x a b) and x simultaneously.

This leads to cases where we failed to reconstruct proofs, since a substitution x -> c, (str.replace x a b) -> d may unintentionally generate the term (str.replace c a b), after which the second substitution fails to apply.