Unification of interfaces across secp256k1 and bls

[pipermerriam]

What is wrong?

It would be nice if py_ecc.secpk256k1, py_ecc.bls12_381 and py_ecc.bn128 all had roughly the same API.

How can it be fixed

A combination of exposing a few un-exposed APIs from secpk256k1 and renaming things.

• expose add and multiply from py_ecc.secp256k1.secp256k1 from py_ecc.secp256k1
• I think py_ecc.secp256k1.secp256k1.inv needs to be renamed/aliased to neg to line up with the bls/bn APIs.
• some other things like field_modulus and curve_order might need to have their equivalent values exposed from secp256k1