Allow more "error" values in try_recv()

[alexcrichton]

This should allow callers to know whether the channel was empty or disconnected
without having to block.

Closes#11087

[bill-myers]

Renaming it to a try_recv_opt that returns Option<Option> seems a better approach, as it doesn't introduce an extra enum type, and follows the simple logic that a nonblocking version of a function (try_recv_opt vs recv_opt in this case) wraps the result of the blocking function in an Option, which allows to convert code between the blocking and non-blocking versions in a straightforward fashion.

Also, Result is supposed to be used for things were it is reasonable to fail! on any error, and it is of course usually not appropriate to fail! on a non-blocking function not returning input yet (that leads to random timing/scheduler dependent failures unless there's some extra serialization to prevent it).

BTW, maybe changing recv_opt to return Result<T, ()> instead of Option might make sense, but that's orthogonal.

[brson]

I think I agree that this should not return a Result. Is it really an error for the channel to be closed? In many cases it's part of the normal program logic.

[brson]

Option<Option<T>> though is not a very nice looking type signature.

[brson]

I'm having a hard time even imagining what Option<Option<T>> means here. Maybe we introduce an enum just for this purpose.

[alexcrichton]

Changed to enum TryRecvResult<T>