build_coinbase api fix and cleanup

[antiochp]

Resolves: #499.
Related: mimblewimble/grin#3416.

V4 cleanup introduced a breaking change to the CbData response.

Old -

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

New -

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

This PR ensures we now produce the expected json when node requests CbData via build_coinbase.

We actually no longer need TransactionV4 and the various related data structures.
This PR cleans this up, allowing the use of a simplified CoinbaseV4 with nested CbOutputV4 and CbKernelV4.

---

I was initially going to add an alternative "compat" ser/deser for output features along the same lines as the exsiting "compat" kernel features. But we do not actually need this as CbData and the version specific CoinbaseV4 is the only place this would be required.

[yeastplume]

All looks good, thanks for cleaning that up. Test failures look like CI's problem, so just triggering them again