Use libsecp256k1 for scalar mult in ECDH in ECIES

[Gustav-Simonsson]

Profiling of a running Go node shows that we spend a significant amount of CPU time in the EC scalar multiplication used when deriving the shared secret in ECDH in ECIES for the encrypted handshake: https://github.com/ethereum/devp2p/blob/master/rlpx.md#encrypted-handshake

The idea is that we should be able to replace the Go code for this multiplication with libsecp256k1 which is highly optimized for this operations.

See: https://github.com/ethereum/go-ethereum/blob/develop/crypto/curve.go#L182
And: https://github.com/bitcoin/secp256k1/blob/master/src/ecmult.h

First attempt at integrating this: https://github.com/Gustav-Simonsson/go-ethereum/tree/scalar_mult

Why the result is different for the same input is unknown - could be we're using the library wrong somehow.

[Gustav-Simonsson]

There is now a PR which adds ECDH at the API level: bitcoin-core/secp256k1#252 Once it's merged we should be able to use it for ECDH in ECIES.

[obscuren]

Sure. Let's integrate that post frontier

[fjl]

This is now done on develop (#1862).