Murisi/multi tx masp vp experiments

[murisi]

Describe your changes

Tried to increase the readability, strength, and generality of the MASP validity predicate whilst decreasing its verbosity. The changes are as follows:

• Transaction inputs and outputs that exceed pre-balances and post-balances respectively are no longer allowed.
• Now use the Sapling value balance in order to verify that movement in and out of the shielded pool is correct
• Now handle all AssetType decoding in one place before starting to check the transparent bundles
• The VP should now support Transactions and unrelated Transfers touching the same addresses/tokens
• Tried to remove the code that gave IBC minting transactions special treatment now that Sapling value balance is used
• Reduced dependency on the multitoken VP by directly checking the correctness of MASP VP balance changes
• Now check that all Transactions and the balancing transaction are sufficiently authorized; prevents malleability

What would still need to be handled/fixed if this code turns out to be relevant:

• Overflows when trying to construct ValueSum::new objects
• Underflows when subtracting Amounts during the value balance computations in verify_sapling_balancing_value
• Add integration tests. Like checking that an unshielding Transaction to Bertha is embeddable inside a Transfer to Christel only if Bertha has signed the entire transaction (in addition to the usual MASP checks).

Indicate on which release or other PRs this topic is based on

#2690

Checklist before merging to draft

• I have added a changelog
• Git history is in acceptable state