Previously we were retrieving nullifiers for notes that were not
marked spent, but not checking against whether the spending
transaction had been included in a block.
`create_spend_to_address` was originally written only for sending to
Sapling addresses. It was later amended to support sending to
transparent addresses, but the assumption about there being a Sapling
output was not removed. This was not an issue for most transactions
because there would be change, but in the case of a z->t transaction
with no change, `create_spend_to_address` would reliably panic.
This commit fixes the bug by setting the output index for transparent
recipients to 0. The `output_index` field of `SentTransaction` is also
documented to correctly reflect its dependency on the type of
`recipient_address`.
The MemoBytes struct is a minimal wrapper around the memo bytes, and only
imposes the existence of null-padding for shorter memos. The only error
case is attempting to construct a memo that is too long. MemoBytes is
guaranteed to be round-trip encodable (modulo null padding).
The Memo enum implements the additional memo rules defined in ZIP 302,
interpreting the contents of a memo (for example, parsing it as text).
Also move dependency on params out of wallet read/write methods.
The result is cleaner because these parameters are only required
for backend-specific encoding and decoding operations.