Deprecated Ethereum sha3 crypto library blocks import of this repository

[PeterCCLiu]

In file curves/altbn128.go, the authors import the deprecated ethereum sha3 crypto library for compatibility.
gosha3 "github.com/ethereum/go-ethereum/crypto/sha3"

This library is removed by ethereum, as stated in here. Now regarding sha3 use, ethereum all moves on to golang.org/x/crypto/sha3 instead.

Keeping this deprecated import causes fail in

• go get github.com/ethereum/go-ethereum/crypto/sha3 within this project
• import . "github.com/Project-Arda/bgls/curves" from another project

Request to delete this import line and function EthereumSum256 in curves/altbn128.go, please. Or I can create a PR for that.

Best,
Peter