Progress on putting ECRecover functionality into contracts

[codingupastorm]

Ethereum contracts increasingly use the ECRecover opcode which returns the address of the private key that signed a message.

This PR introduces that functionality and was just starting to test it. It needs more work before being merged.

The new functionality was enabled in the Stratis.SmartContracts v1.2.2-dev package.

It needs:

• An integration test deploying a contract that uses this opcode and then testing that it gets the correct address and can verify that something was signed off-chain
  -The Stratis.SmartContracts package 1.2.3 to be released with this enabled.
• Cirrus nodes to be upgraded (hard fork) to the new Stratis.SmartContracts package.

[codingupastorm]

Stratis.SmartContracts 1.2.2 is here stratisproject/Stratis.SmartContracts#5