Use uninterpreted functions for division

SMT bitvector division (`bvudiv`, `bvsdiv`, `bvurem`, `bvsrem`) is expensive for solvers. Halmos uses **uninterpreted functions** during symbolic execution, deferring exact division semantics to a refinement phase. This dramatically speeds up path exploration while remaining sound.

Start by tracing hevm's symbolic-execution and refinement phases, since the issue names no files or tests. Determine where bvudiv, bvsdiv, bvurem, and bvsrem are handled; done means symbolic execution uses uninterpreted functions while refinement preserves exact division semantics and soundness.

Written by the indexing model from the issue text.