Provides a (very) simple async function combinator.
If f :: A -> B, then async!f :: A -> Future!B.
Suppose f(a) = b and async!f(a) = c.
async!f will execute f in a separate thread when invoked.
While f is being computed, c.pending.
If f succeeds, c.completed and b == c.result.
If f throws, c.failed.
Attempting to read c.result before c.completed is an error.
async!f can be made blocking at any time with c.await.
For convenience, c.await returns c so that c.await.result == c.result.
Note that, for async!f, f must be public so that the async template can see it.
To lift a non-publicly-visible function g :: A -> B, use the non-template async(g, a).