sygus: Eliminate calls to Rewriter::rewrite. (#7142)

sygus: Eliminate calls to Rewriter::rewrite. (#7142)

This derives sygus unification utility objects from EnvObj where
necessary. There's one remaining occurrence of Rewriter::rewrite in
sygus_unif_rl.cpp, which is a bit tricky to address and thus subject to
a future PR.