chore: decimal module macro cleanup (#123791)

* protect macros expansion via `do { ... } while (0)` constructions in `_decimal.c`

* Use public macro `Py_UNUSED`

This replaces the usages of the `UNUSED` macro which
was not consistent with the `Py_UNUSED` macro itself.

In addition, this amends the parameter names so that
they match their semantic meanings.

* Remove redundant `PyCFunction` casts