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.

tests/unix/extra_coverage.py.exp

@@ -38,7 +38,7 @@ ementation
 0
 0
 # runtime utils
-TypeError: can't convert str to int
+TypeError: unsupported type for : 'str'
 TypeError: unsupported types for : 'str', 'str'
 Warning: test
 # format float