perf(kernel/replace_visitor): use more expensive/precise cache

It does not use pointer equality, but structural equality

src/kernel/replace_visitor.h

@@ -16,7 +16,7 @@ namespace lean {
 */
 class replace_visitor {
 protected:
-    typedef expr_map<expr> cache;
+    typedef expr_bi_struct_map<expr> cache;
     cache   m_cache;
     expr save_result(expr const & e, expr && r, bool shared);
     virtual expr visit_sort(expr const &);