Store decrypted scanning results in the scanner storage

[teor2345]

Motivation

We want to store the decrypted results of scanning in the scanner storage.

Complex Code

These types look pretty complicated, so it might help to use HashMap's entry API.

It would be helpful to add type aliases for SaplingSpend and SaplingOutput, so we can change them later.

Suggested Design

In sapling_spend_results, when Nf is () it should become None.

    /// The new outputs created in each transaction, for each scanning key. 
    sapling_spend_results: HashMap<String, HashMap<Hash, WalletSaplingSpend>>,

    /// The spent outputs by each transaction, for each scanning key.
    /// If the scanning key is an individual viewing key (without a nullifier deriving key), there are no nullifiers in those results.
    sapling_spend_results: HashMap<String, HashMap<Hash, WalletSaplingOutput<Option<Nullifier>>>>,

Originally posted by @teor2345 in #7942 (comment)

[mpguerra]

We won't be doing any more work on the scanner