Confidential package

[altafan]

To support the blinding/unblinding of elements utxos we need to create the functions that actually use the methods of the secp256k1-zkp lib.
We can take inspiration by the js module we created in the liquidjs-lib library.

We should then create a condifential package that collects the methods for the following purposes:

• Creating a nonce hash from a keypair
• Unblinding a utxo given a blinding private key.
• Creating a final value blinding factor for the last transaction output.
• Creating an asset commitment.
• Creating a value commitment.
• Creating a range proof.
• Creating a surjection proof.

All the points in this bullet list will be expanded in dedicated issues, but all of them represent a necessary step in order to implement the pset's Blinder role of #45.

[tiero]

I guess could be closed? @altafan