spawn_unsafe should require that F: Send

[jonhoo]

The spawn_unsafe method in Crossbeam should include a bound on the function f: F that it is also Send. Otherwise, you may end up calling functions that access non-thread-safe things like UnsafeCell. This also brings the definition more in line with that of std::thread::spawn. Specifically, the signature should be:

pub unsafe fn spawn_unsafe<'a, F>(f: F) -> JoinHandle<()> 
where F: FnOnce() + Send + 'a

[aturon]

The bounds were deliberately left off to provide a maximally flexible, unsafe spawning function -- i.e., to cover even the case where you wanted to work with a non-Send closure unsafely.

That said, the flexibility isn't needed for scoped threads, and of course does add to the overall danger of this function. Arguably, this treatment of Send might be better accomplished through a newtype wrapper anyway. So I'm open to adding the bound.

[jonhoo]

An alternative might be to provide two unsafe functions instead of one, where one requires Send, and its use is recommended where possible.

[aturon]

Given that there's not a pressing need for a version without Send, I'd accept a PR that just added Send to the current method.

[jonhoo]

PR submitted: #55.