Lifetime based Handles

[ishitatsuyuki]

Currently the Core is using a reference counter to create handles. However, I believe this can be elided with lifetimes.

Implementation notes:

• spawn() should take 'a (outlives Core) instead of 'static.
• technically Remote doesn't require a lifetime (it uses mpsc), but I think it should have PhantomData for consistency.
• hacks required for CURRENT_LOOP.

[alexcrichton]

Is a reference count a problem?

[carllerche]

Down the road, I will probably try exploring making Core generic over the task type (though the primary API will remain without the generic). This would primarily be to be able to remove the required alloctions but could also potentially be used for adding lifetimes.

Though, again it isn't high priority and not an immediate concern.