Minor API pain-points

[sanket1729]

These are things that are easy to workaround downstream, but would one-liner if we had upstream support. I will update the list as I find more things:

1. Direct API for calculating assetid and token id directly from prevouts during initial issuance. This currently a multi step process which requires dealing calling the AssetId::generate_asset_entropy() API.
2. Display implementations of Explicit Asset should add the 0x01 prefix
3. Add FromStr implementation for confidential::Asset/Value/Nonce. Currently, we need to decode a bytes and call encode::deserialize. It would be great to have methods for a) from_slice/to_vec() b) Display/FromStr c) FromHex/ToHex.
4. Implement auto-derives for Error data structures. For example: UnBlindError.
5. Separate out TxOut::unblind API into two APIs. TxOut::unblind_from_prf. This will also it to re-use it for psbt logic too