fix: reduce cost of signer key-search algorithm with pre-compute table

[bhgomes]

Signed-off-by: Brandon H. Gomes bhgomes@pm.me

Before this PR, the signer key-search algorithm would re-compute every viewing key within the search range on every insertion of a UTXO. Now, we use a dynamic program that replaces the viewing key computation with a lookup if the key has already been computed. To prevent run-away growth of this table we have a max capacity set to 16 times the GAP_LIMIT (currently set to 20), a very unlikely limit to be reached except for implementation errors or purposeful memory attacks.

This PR also adds some utility functions for B-Trees.

Focus Files:

• manta-accounting/src/wallet/signer.rs
• manta-accounting/src/key.rs

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[ghost]