API surface of the `Wallet` type

[thunderbiscuit]

After taking some time to look at what parts of the Wallet API were available/not available/useful for the bindings, many methods stood out. This issue will track them. Note that not all of those are top priority, but most of them we figured would be useful and not too costly to include either in the first version of the 1.0 or in a future feature release (1.1, 1.2, etc.)

• [x] Wallet::peek_address
• [ ] Wallet::cancel_tx
• [x] Wallet::next_derivation_index
• [ ] Wallet::derivation_of_spk
• [ ] Wallet::descriptor_checksum
• [ ] Wallet::finalize_psbt
• [ ] Wallet::get_utxo
• [x] Wallet::list_unused_addresses
• [x] Wallet::mark_used
• [x] Wallet::next_unused_address
• [ ] Wallet::policies
• [ ] Wallet::public_descriptor
• [x] Wallet::reveal_addresses_to
• [ ] Wallet::unbroadcast_transaction

[rustaceanrob]

What I would consider high priority:finalize_psbt, cancel_tx, public_descriptor, descriptor_checksum. I can hack at those if others agree they should be included in 1.0