make create_transaction() read only. attempt 3.

[dan-da]

Addresses #122. Obsoletes #159.

This is my 3rd attempt to refactor create_transaction(). Perhaps 3rd try is the charm.

For original background motivation and discussion, see previous attempts: #136 and #159.

Some goals of this attempt are:

• address Alan's comments in 159.
• make the code simpler and cleaner to read, understand, maintain.
• make UtxoNotification variants symmetric.
• keep create_transaction() simple for caller to use, but also flexible.
• lay some groundwork for onchain symmetric key notification
• abstract over key/address type in public APIs.
• prepare for using derived key indexes in a future commit.

Changes since #159:

highlights / key changes:

• input utxos are now automatically matched to the appropriate spending key. previously that was a TODO.
• abstract SpendingKey and ReceivingAddr in create_transaction() and all RPC APIs. This should make it possible to add new address types without major refactors post-launch.
• add create_raw_transaction() for fine-grained control over inputs and outputs, including change. (Same api exists in bitcoin.)
• all create_transaction() related code is now simpler and cleaner, at least to my eye, with significant functionality moved into TxInput, TxInputList, TxOutput, TxOutputList.
• create_transaction() return type is now just a Transaction, not a tuple.
• change spending key is now a param to create_transaction() in order to keep it &self. (see discussion below).

rpc_server:

• own_receiving_address --> next_receiving_address
• SpendingKey -> SpendingKeyType in rpc APIs
• ReceivingAddress -> ReceivingAddressType in rpc APIs
• adapt send_to_many() to create_transaction() changes. (simpler now)

global_state:

• simplify TransactionDetails, and is now private struct.
• eliminated several private helper fn
• simplified create_transaction family of fns with TxInputList, TxOutputList
• added doc comments
• add generate_tx_outputs() public helper method
• create_transaction() args changed:
  • receiver_data --> tx_outputs
  • tx_outputs is now &mut, and may have change output appended for caller's use.
  • add change_spending_key
• create_transaction() return type is now just a Transaction, not a tuple.
• caller must pass a change spending key into create_transaction() because next_unused_generation_spending_key() takes &mut self.
• added create_raw_transaction() for fine grained control over inputs and outputs
• simplify create_transaction() helper fns.

wallet:

• add abstract SpendingKeyType and ReceivingKeyType
• add wallet_state::find_spending_key_for_utxo()
• wallet_state::allocate_sufficient_input_funds_from_lock() now returns matching SpendingKeyType with each input. (fixes prev TODO)

transaction:

• add TxInput, TxInputList, TxOutput, TxOutputList
• add UtxoNotifyMethod, UtxoNotification

other:

• UtxoReceiver -> TxOutput

Explanations of things that may not be obvious:

1. preparing for derived keys. Presently some functions call WalletSecret::nth_generation_spending_key(&self, counter: u16), passing 0. I've now added ::next_unused_generation_spending_key(&mut self), which presently always uses index 0 but in the future will increment a stateful counter. The difficulty is that it must take &mut self, so it cannot be called from within GlobalState methods that take &self, such as create_transaction(). So create_transaction() now requires the caller to provide the next unused spending key.

2. let me know if there's something else. I put a lot of code comments, but let me know if anything is unclear.

[dan-da]

I went ahead and merged this. @Sword-Smith if you have any concerns, pls let me know.