Implement Confidential Transactions

[HarryR]

As per:
https://github.com/mimblewimble/grin/blob/master/doc/intro.md
https://fc17.ifca.ai/bitcoin/papers/bitcoin17-final41.pdf

The components necessary would be:

• Range proof (base 4? e.g. Back-Maxwell range proof) to verify the value
• Pedersen commitment to amount
• Signature provided by the blinding key

[jskskjnekd]

Among three methods (hiding receiver/hiding sender/hiding transaction amount), range proof is of the most advanced (but also of the most difficult) one.