Feature: Propagate output type into custom executor closure

[jlizen]

Right now we type erase the input and output future that we send to our custom executor closure by wrapping everything in a cancellable oneshot channel.

This is the simplest API for callers and generally a reasonable default. But, there are use cases that might benefit from preserving some type information.

For instance: Perhaps there are multiple future types, and some should be weighted heavier for acquiring concurrency permits.

One initial thought is, expose a Any<PhantomData<T>> input into the closure, where T is the output type of the input future. Maybe via an alternate setting of the existing executor.

There might also be benefit in another 'turnkey' strategy that allows selecting between multiple other strategies based on input future type, rather than requiring the full fledged custom executor?