Make uint256 and uint160 implement IEquitable (#671)
Make uint256 and uint160 implement IEquitable (#671)

* Make uint256 and uint160 implement IEquatable interface

* Make IEquitable implementation more performant

* Use is null instead of == null

NicolasDorier/NBitcoin,MetacoSA/NBitcoin,MetacoSA/NBitcoin