Improper Abstractions regarding TransactionDetails and UtxoNotification

[Sword-Smith]

Struct TransactionDetails contains info needed for making a transaction in the context of interfacing with the client's state. (It now lives in a file name transaction_details.rs by the way.)

However, TransactionDetails has no field for PublicAnnouncements and perhaps it should. It does have a field for TxOutputs which have a UtxoNotification field, and if this field is set to the variant OnChain then the associated data is a PublicAnnouncement. With this architecture, it seems as though it is impossible to create transactions whose public announcements do anything other than notify receivers of UTXOs. This might be the case in the immediate future, but in the more distant future we definitely want to open up PublicAnnouncements to whatever the transaction initiator wants to say, even if that is not used for securely transmitting a ciphertext to someone. An example comes to mind: a disputed closure of a Lightning channel would require such a public announcement.

Here is a possible solution: variant UtxoNotification::OnChain contains a plaintext that is not encrypted yet, instead of the ciphertext cast to public announcement.

Co-authored-by: @aszepieniec

[dan-da]

create_transaction(), TransactionDetails and UtxoNotification are changed significantly in #185. TransactionDetails is replaced by TransactionParams.

I suggest that you complete the merge against master as-is, and then we can evaluate/discuss with regards to impl in #185. The current master is sort of an intermediate state. Fixing it (whatever that means) now would just create more merge headaches for #185.