This code uses some kludges (discussed with Ian previously and
hopefully well documented here) to get a type-identifier for each
type in a const context. It then defines a macro to declare a
type-erased versions of a concrete implementation functions, and
register those implementations to be called later.
We will probably want to tweak a bunch of this code as we move ahead.
Ordinarily you can cancel a future just by dropping it, but we'll
want the ability to cancel futures that we no longer own (because we
gave them to a `FuturesUnordered`).
Nick Mathewson