Migrate from btcd/btcec to go-ethereum/crypto/secp256k1
Abstract calls on secp256k1.S256()
Change newRand approach, use ecdsa.GenerateKey underneath
Add check of size of mBlinded & k when blind signing

Previous to this commit there was the implementation of "[An Efficient Blind Signature Scheme Based on the Elliptic Curve Discrete Logarithm Problem](http://www.isecure-journal.com/article_39171_47f9ec605dd3918c2793565ec21fcd7a.pdf)" paper by by Morteza Nikooghadama & Ali Zakerolhosseini.

This commit adds the implementation of "[New Blind Signature Schemes Based on the (Elliptic Curve) Discrete Logarithm Problem](https://sci-hub.do/10.1109/ICCKE.2013.6682844)" paper by Hamid Mala & Nafiseh Nezhadansari.