consider removing receiver_preimage field from ExpectedUtxo.

[dan-da]

I'm just moving this lengthy comment from code (expected_utxo.rs) into github. The code will reference this issue.

/// ### about `receiver_preimage`
///
/// The `receiver_preimage` field in expected_utxo is not strictly necessary.
/// It is an optimization and a compromise.
///
/// #### optimization
///
/// An `ExpectedUtxo` really only needs `utxo`, `sender_randomness` and
/// `receiver_identifier`.  These match the fields used in `PublicAnnouncement`.
///
/// However it then becomes necessary to scan all known wallet keys (of all key
/// types) in order to find the matching key, in order to obtain the preimage.
///
/// improvement: To avoid scanning a map of \[`receiver_identifier`\] -->
/// `derivation_index` could be stored in local wallet state for all known keys.
///
/// However no such map presently exists, and so the most efficient and easy
/// thing is to include the preimage in [ExpectedUtxo] instead of a
/// `receiver_identifier`.  This buys us an equivalent optimization for little
/// effort.
///
/// #### compromise
///
/// Because the preimage is necessary to create an [ExpectedUtxo]
/// `create_transaction()` must accept a `change_key: SpendingKey` parameter
/// rather than `change_address: ReceivingAddress`.  One would normally expect
/// an output to require only an address.
///
/// note: one *would* expect the SpendingKey parameter for inputs.
///
/// Further if [ExpectedUtxo] and `PublicAnnouncement` use the same fields then
/// they can share much of the same codepath when claiming.  At present, we have
/// separate codepaths that perform largely the same function.
///
/// We may revisit this area in the future, as it seems ripe for improvement.
/// In particular wallet receiver_identifier map idea indicates it is possible
/// to do this efficiently.  As such it may be best to implement at least the
/// scanning based approach before mainnet.
///
/// A branch with an implementation of the scanning approach exists:
/// danda/symmetric_keys_and_expected_utxos_without_receiver_preimage
///
/// At time of writing many tests are not passing and need updating with the new
/// field.