py/modbuiltins: Implement abs() by dispatching to MP_UNARY_OP_ABS.

This allows user classes to implement __abs__ special method, and saves
code size (104 bytes for x86_64), even though during refactor, an issue
was fixed and few optimizations were made:

* abs() of minimum (negative) small int value is calculated properly.
* objint_longlong and objint_mpz avoid allocating new object is the
  argument is already non-negative.

py/runtime.c

@@ -231,6 +231,15 @@ mp_obj_t mp_unary_op(mp_unary_op_t op, mp_obj_t arg) {
                 } else {
                     return MP_OBJ_NEW_SMALL_INT(-val);
                 }
+            case MP_UNARY_OP_ABS:
+                if (val >= 0) {
+                    return arg;
+                } else if (val == MP_SMALL_INT_MIN) {
+                    // check for overflow
+                    return mp_obj_new_int(-val);
+                } else {
+                    return MP_OBJ_NEW_SMALL_INT(-val);
+                }
             default:
                 assert(op == MP_UNARY_OP_INVERT);
                 return MP_OBJ_NEW_SMALL_INT(~val);