When we start a fiber via the _fiber_start() API family, we don't
get back a handle on the created fiber. The fiber identifier is
actually the start of the fiber's stack. This hasn't been a problem
until now since no API requires a handle on the fiber, except one,
fiber_delayed_start_cancel(): that API is part of a pair, where the
other API, fiber_delayed_start() starts the fiber and returns a
handle.

However, Jukka asked me an API could be created that cancels a
fiber_sleep() call, something like fiber_wakeup(). The
implementation of such an API is very simple, but it requires a
handle on the fiber we want to wake up. This in turn requires the
signature of the _fiber_start() family to return a handle to the
fiber that gets started.

The signature of _fiber_start() et al. would then change from a void
return type to a void * return type.

Objections, comments, etc ?

No comments -> no objects -> perhaps we can continue with this route
then?