This package provides a higher-level interface over threads, in which
an Async a is a concurrent thread that will eventually deliver a value
of type a. The package provides ways to create Async computations,
wait for their results, and cancel them.

Using Async is safer than using threads in two ways:

    When waiting for a thread to return a result, if the thread dies
    with an exception then the caller must either re-throw the
    exception (wait) or handle it (waitCatch); the exception cannot be
    ignored.

    The API makes it possible to build a tree of threads that are
    automatically killed when their parent dies (see withAsync).