fix(frontends/lean/elaborator): add missing synthesize

We added for the following reasons:
1- It should mimic the behavior of `visit_lambda` and `visit_pi`.
2- It minimizes the number of auxiliary metavariables that need to be
   created when we execute `locals.mk_lambda(new_body)`. In Lean3,
   it would minimize the number of delayed abstractions.

src/frontends/lean/elaborator.cpp

@@ -3523,7 +3523,7 @@ expr elaborator::visit_let(expr const & e, optional<expr> const & expected_type)
     save_identifier_info(l);
     expr body      = instantiate_rev_locals(let_body(e), locals);
     expr new_body  = visit(body, expected_type);
-    /* TODO(Leo): add synthesize here? */
+    synthesize();
     expr new_e     = locals.mk_lambda(new_body);
     return new_e;
 }