[Part 1] No-Op Refactors from optimize-ram-in-scan-states

[martyall]

This PR represents the no-op work from @georgeee's work on georgeee/optimize-ram-in-scan-states

In reverse order, this is given by

• 506ae09 Move Transaction_type to a standalone module
• fc9a426 Change function latest_ledger_proof_{and_ -> }txs
• 4932bd8 Simplify type of fill_work_and_enqueue_transactions
• 48257a0 Introduce latest_ledger_proof and use where applicable
• 84da5d1 Rename latest_ledger_proof{ -> _and_txs}
• 8cfaa17 Remove unused functions
• a12c957 Add map_result for the scan tree <- probably not used, to be dropped
• 57019e5 Add comment about future modifications of scan state
• f8b5ce4 Simplify type of init_stack in witness
• 803b42d Add previous_protocol_state_body_opt to witness
• 71836a9 CAUTION DON'T MERGE: simplify transaction witness
• d0b5fca Don't source accounts created from scan state
• 5ad7878 Remove status_of_applied
• 2cf32d5 Remove transaction_of_applied
• 0eb863a Simplify all_work_pairs
• 9b71088 Simplify One_or_two.group_list
• b35745f Add some comments
• 06e226d Introduce Tagged types for witness and ledger proof
• 52b83de Persist witnesses and proofs on disk

This were all cherry-picked over directly, except for d0b5fca where I made [this change](https://github.com/MinaProtocol/mina/pull/18162/commits/de3b2e55f3178614ebfd0999b544f4591e29681b#diff-7b6a8e274faf8286a1c899878cf16bdec3bb8a84297f75f8bfca4a217819c9dcR138-R148)

[martyall]

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[martyall]

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[martyall]

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