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/runtime0.h

@@ -50,6 +50,7 @@ typedef enum {
     MP_UNARY_OP_NEGATIVE,
     MP_UNARY_OP_INVERT,
     MP_UNARY_OP_NOT,
+    MP_UNARY_OP_ABS, // __abs__
     MP_UNARY_OP_SIZEOF, // for sys.getsizeof()
 } mp_unary_op_t;