Use custom hash function for eth1 ecdh

[arnetheduck]

We currently extend upstream libsecp256k1 to add a secp256k1_ecdh_raw function - since this was implemented, upstream has added the ability do do custom hash functions that could be used to implement this ethereum-specific way of deriving keys - as such, we can stop patching upstream and move the logic into nim-eth.

Upstream discussion: https://github.com/bitcoin-core/secp256k1/issues/352

Implementation blueprint: https://github.com/bitcoin-core/secp256k1/blob/master/src/modules/ecdh/tests_impl.h#L18-L25