custom hashing functions

[scgbckbone]

Provide a way to accept user defined hash functions where it is possible (ECDH, schnorr)

[scgbckbone]

https://github.com/scgbckbone/python-secp256k1/compare/coldcard_ecdh this branch adds support for ECDH custom hashfp. I'm not sure about function passing behavior.

Currently it expects users to import additional ctypes function signature class, wrap custom function in it and pass it to ecdh:

from pysecp256k1.ecdh import ECDH_HASHFP_CLS
from mywork import my_custom_ecdh_hashfp

custom_hashfp = ECDH_HASHFP_CLS(my_custom_ecdh_hashfp)
ecdh(seckey, pubkey, hashfp=custom_hashfp)

Second option is to let users just pass python function and wrap it to ECDH_HASHFP_CLS internally. I consider this to be easier but also more error prone as user may have no idea how ECDH_HASHFP_CLS looks in the first place and therefore implementation of custom function may be wrong.