Optimized ecmul on secp256k1

[k06a]

Recently Vitalik Buterin proposed to abuse ecrecover to compute hashed result of ecmul:
https://ethresear.ch/t/you-can-kinda-abuse-ecrecover-to-do-ecmul-in-secp256k1-today/2384

I had implemented his idea: https://github.com/1Address/ecsol/blob/master/contracts/EC.sol

It uses about 32K gas to call ecmulVerify.