Remove extra copy when creating public keys and signatures

[James-E-A]

Should get rid of the non-lazy """casts""" from FFI-Owned Memory to Python Bytes Objects

Obviously this is a nearly irrelevant performance improvement, premature optimization yadda yadda, but it's still something that's clearly the wrong thing to do, so we should do the right thing at some point.

[James-E-A]

Not gonna be so easy.

What I tried:

• A function with signature PyObject* f(PyObject* m, PyObject* sk); apparently cannot be exposed by CFFI.
• A function with signature PyObject* f(uint8_t* m, size_t mlen, uint8_t* sk) apparently does not allow actually "exfiltrating" the created object. Even ffi.from_handle(f(…)) raises TypeError: expected a 'cdata' object with a 'void *' out of new_handle(), got 'PyObject *'

I think if we want to construct the PyBytes object manually to avoid the extra copy, we'll have to ditch CFFI and just do things raw.

[James-E-A]

As much as I'd like to ditch CFFI due to its throwing a wrench in #1, that would be a lot of work I'm not up for.

I'm closing this as "I won't fix it right now", but I would be glad to re-open this if anyone wants to help.