[#841] Adjust to sapling changes in jakarta

Description

Excerpt from the Jakarta release notes:

The existing type sapling_transaction is renamed sapling_transaction_deprecated and is deprecated. Existing onchain contracts are automatically converted. A new Michelson type sapling_transaction and an overload of the instruction SAPLING_VERIFY_UPDATE have been added to fix the malleability problem of the old instruction (see Sapling integration). (MRs tezos/tezos!4670 (merged), tezos/tezos!4589 (merged))

• Since sapling_transaction_deprecated can't be used for new contracts (it's rejected by tezos-client typecheck and tezos-client originate), we won't be supporting it in morley.
• Which means we only need to make sure SAPLING_VERIFY_UPDATE works on the "new" sapling_transaction type.
• This MR also makes sure morley fails with a neat error message when a user attempts to parse/unpack a contract with sapling_transaction_deprecated

As part of this MR, I've also uncovered and fixed some bugs in the deserializer. Details in the commits' descriptions.

Related issue(s)

Resolves #841 (closed)

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