Rewrite std::sync::TaskPool to be load balancing and panic-resistant

The previous implementation was very likely to cause panics during
unwinding through this process:

- child panics, drops its receiver
- taskpool comes back around and sends another job over to that child
- the child receiver has hung up, so the taskpool panics on send
- during unwinding, the taskpool attempts to send a quit message to
  the child, causing a panic during unwinding
- panic during unwinding causes a process abort

This meant that TaskPool upgraded any child panic to a full process
abort. This came up in Iron when it caused crashes in long-running
servers.

This implementation uses a single channel to communicate between
spawned tasks and the TaskPool, which significantly reduces the complexity
of the implementation and cuts down on allocation. The TaskPool uses
the channel as a single-producer-multiple-consumer queue.

Additionally, through the use of send_opt and recv_opt instead of
send and recv, this TaskPool is robust on the face of child panics,
both before, during, and after the TaskPool itself is dropped.

This TaskPool uses an additional "monitor" task start new child
tasks to replace those that panic.

Due to the TaskPool no longer using an `init_fn_factory`, this is a

[breaking-change]

otherwise, the API has not changed.

If you used `init_fn_factory` in your code, and this change breaks for
you, you can instead use an `AtomicUint` counter and a channel to
move information into child tasks.
Requires the impl Drop for Sentinel to need #[unsafe_destructor], but that's fine in this case.