Merge Funcky.Async back into Funcky

[bash]

Considering that the System.Linq.Async package is going to be part of the BCL I propose that we merge Funcky.Async back into Funcky.

I haven't had time to dig into the specifics, however one thing that I have noticed is that the BCL doesn't use the *Await, *WithCancellation convention e.g. the version of Select that takes an async selector is called Select, not SelectAwait.