"com" and "prf" in coinbase from wallet

[antiochp]

Resolves: mimblewimble/grin-wallet#499

We ask the wallet for a CbData (output and kernel for block reward).

The output looks like this -

	"output": {
		"com": "08fe198e525a5937d0c5d01fa354394d2679be6df5d42064a0f7550c332fce3d9d",
		"features": 1,
		"prf": "9d8488fc..."
	}

It used to look like this -

	"output": {
		"commit": "08fe198e525a5937d0c5d01fa354394d2679be6df5d42064a0f7550c332fce3d9d",
		"features": "Coinbase",
		"proof": "9d8488fc..."
	}

This PR adds an alias to both commit and proof via a custom CbOutput.
For convenience we ignore the output features (and kernel features) as this wallet api will only return coinbase specific output and kernel.
This code is only used for this single wallet api call.

Tested locally on usertesting and confirmed the node can talk to the wallet successfully (both on master).

[antiochp]

Closing. We should fix this on wallet side to maintain compatibility between old nodes and new wallet.

Resolved in - mimblewimble/grin-wallet#503