A future value is a value that will become knowable only later. This
module gives a way to manipulate them functionally. For instance,
a+b becomes knowable when the later of a and b becomes knowable.
See http://en.wikipedia.org/wiki/Futures_and_promises.
Primitive futures can be things like /the value of the next key you
press, or the value of LambdaPix stock at noon next Monday/.
Composition is via standard type classes: Functor, Applicative,
Monad, and Monoid. Some comments on the Future instances of
these classes:
 Monoid: mempty is a future that never becomes knowable.
a mappend b is whichever of a and b is knowable first.
 Functor: apply a function to a future. The result is knowable when
the given future is knowable.
 Applicative: pure gives value knowable since the beginning of
time. '(<*>)' applies a future function to a future argument.
Result available when both are available, i.e., it becomes knowable
when the later of the two futures becomes knowable.
 Monad: return is the same as pure (as always). (>>=) cascades
futures. join resolves a future future into a future.
The current implementation is nondeterministic in mappend for futures
that become knowable at the same time or nearly the same time. I
want to make a deterministic implementation.
See Data.SFuture for a simple denotational semantics of futures. The
current implementation does not quite implement this target semantics
for mappend when futures are available simultaneously or nearly
simultaneously. I'm still noodling how to implement that semantics.
