Interrupt Transaction_pool.apply when the ledger is detached

[mrmr1993]

This PR

• adds a detach_signal method to ledgers triggered when
  • the ledger is closed
  • the ledger is a mask, and its parent is removed without the intention to replace it
• adds Interruptible.Deferred_let_syntax
  • this exposes a Let_syntax module that works with ppx_let to consume Async.Deferred.ts and return Interruptible.ts
  • this allows us to make an async function interruptible without having to modify any of the code
  • this also supports let%map_open/let%bind_open, where the rhs of the binding can use the Deferred.t monad as normal.
• converts Transaction_pool.apply to interrupt when its ledger emits a detach signal
  • this combines detach_signal and the Interruptible.Deferred_let_syntax to trigger the interrupt with minimal code changes

Open question: should we re-run the apply when it has been interrupted? As currently written, we treat it as a bad diff.

Checklist:

• Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• All tests pass (CI will check this if you didn't)
• Serialized types are in stable-versioned modules
• Does this close issues? List them:

Closes #6946