feature(fiber): add Cancel.make_child for linked cancellation

[rgrinberg]

Summary

Adds Fiber.Cancel.make_child, a way to link cancellations together:

val make_child : t -> t

make_child parent returns a new cancellation that is fired whenever parent is
fired (but not the other way around). This makes it possible to build trees of
cancellations where cancelling a parent propagates to all of its descendants.

Details

• A Not_cancelled cancellation now tracks its children and its parent.
• fire and fire' walk the child tree, so firing a node fires its whole
  subtree. The order in which a parent's and its children's handlers run is
  unspecified.
• When a node fires it detaches itself from its parent, so short-lived children
  attached to a long-lived parent do not accumulate. (A child is otherwise
  retained until it or its parent is fired, or the parent is collected.)
• with_handler's behaviour is unchanged.

Tests

src/fiber/test/cancel_tests.ml gains three expect tests covering:

• cancelling a parent triggers a child's cancellation handler,
• a child created from an already-fired parent is born cancelled,
• cancelling a child does not cancel its parent.