Add `spawn_fn` to Remote.

[carllerche]

This issue currently represents the task of unifying "spawning" functions across Handle, Remote and potentially other executors (like CPU pool).

Currently Handle::spawn takes a future where as Remote::spawn takes a fn.

Also, there are other questions to consider, such as #77

Original issue

It looks like spawn_fn already exists on Handle.

https://tokio-rs.github.io/tokio-core/tokio_core/reactor/struct.Handle.html#method.spawn_fn

[carllerche]

Oh, looks like Remote::spawn already is a fn! sorry :(

[carllerche]

Maybe Remote::spawn should be renamed Remote::spawn_fn for consistency?

[carllerche]

Another thought, maybe Remote::spawn could return a Future of the return value (like thread::spawn). There could be an additional variant remote::fire(....) that doesn't return a future?

[alexcrichton]

Yeah I think we should rename for consistency, although we've avoided in the past having a return value of a future for two reasons:

1. The semantics elsewhere when you drop the receiver are that the computation is canceled, which we can do but may not be the best default?
2. This can always be easily emulated with oneshot.

[carllerche]

1. We should probably revisit this. I liked where it was left off where cancellation is the default, but there is a detach fn that prevents the cancellation.
2. I've needed to add the oneshot boilerplate every time (as far as I can tell looking back), which makes me think that it would be nice to include it by default (with another function that does't maybe, like fire).