Pick a blinding key/blinding factor standard compatible with hww++

Currently we kind of YOLO how these keys are derived, in that a backed up `wallet.dat` will properly restore funds, but these schemes are not cross-compatible with devices such as hardware wallets, and wallets that may not allow raw privkey export.

So I think for blinding derivation stuff we basically have:

1) signing keys on some path, hardened or unhardened paths from some hardened parent
2) master blinding key, which is actually Hash(some_pubkey_in_odd_hardened_path). This allows export of master blinding privkey for auditing purposes even for hardware wallets(xpub and xprv together to track funds in a non-custodial manner). This does mean an unlocked hww will cough up blinding pubkeys on a malicious host without intervention.
3) asset/value blinding factors: Some odd derivation path with hardened part, either take a single subkey and HMAC it with `txid:nOut`, or chunk up the txid and use it as a few normal derivation indices.