Consider switching Arc<Unpark> to an opaque type

[carllerche]

This issue is mostly a placeholder for a discussion leading up to 0.2.

Currently, Arc is used to pass around the unpark handle. This API has a hard requirement on Arc which means that an allocation is required even if it isn't strictly necessary.

I propose switching that to an opaque struct, something like UnparkHandle which would allow the internal representation to change in the future.

There could be a conversion from Arc as I think we would need to maintain that indefinetly.

Relates to #430

[alexcrichton]

I'd be surprised if we could do this, but I'd be willing to try!

[alexcrichton]

Done in #436!