Rework Interruptible monad

[mrmr1993]

This PR rewrites the interruptible monad to behave correctly. In particular, this changes the semantics around binding so that

let%bind x = (* this gets the interrupt *) in
let%bind y = (* while running this *) in
let%bind z = f y in
(* ... *)

has the same behaviour as

let%bind x = (* this gets the interrupt *) in
let%bind z =
  let%bind y = (* while running this *) in
  f y
in
(* ... *)

This was not the case in either the original version or the tweaked implementation in #6880. Presumably this was causing issues in Integration_test_cloud_engine.Stack_driver_log_engine.pull_subscription_in_background, which calls itself infinitely in this nested-bind style.

This also adds unit tests for these cases, and fixes the existing unit test (which was equivalent to Async.after (Time.Span.of_ms 130.)).

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 #300.

[psteckler]

might be nice to have a comment that says this provides bind, map, and return

[psteckler]

each other

[psteckler]

might be nice to say what a "signal" is in this context (it's not a Unix signal)