test: add `Sym.simp` arithmetic normalizer benchmarks

This PR adds benchmarks for the `arith` simproc of `Sym.simp`: long linear sums, polynomial expansions over `Int`, `Nat`, commutative (semi)rings and noncommutative rings, fields, rings with a nonzero characteristic, relations, and many small goals.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
6 files changed