* Allow a Py_buffer as data for Rules_match

This makes rules matching compatible with data objects
`PyArg_ParseTuple` does not consider read-only (even though they might
actually be), such a memoryviews. The main change is replacing the `s#`
formatter with `s*` and replacing the `(pointer, length)` pair with a
`Py_buffer` object accordingly. Additional care must be taken to release
the `Py_buffer` on every error path.

* Rules_match: zero-initialize data

PyArg_ParseTupleAndKeywords does not initialize optional fields unless
they are passed, which means we need to zero-initialize the data buffer
to be sure the later NULL checks always work.

This commit also gets rid of the unneeded has_data flag.

* Add test for matching on a memoryview