Extract secp256k1 code into a python package

[real-or-random]

"secp256k1-proto" sounds good to me

[real-or-random]

It's not yet entirely clear to me how this should like in the end.

• Will we include the full code of the package in the BIP? Or just refer to it, possibly pinned to some commit. The latter sounds cleaner.
• The package should at least have docstrings for all objects we import, to clarify the operations we rely on (like in BIP340, where we define stuff like lift_x). But then, if we don't include the full contents of the package, then the BIP is technically speaking not fully self-contained... Is that a problem? In theory, we could also run pydoc on the package and include the output in the BIP. But that sounds a bit like overkill.