Add Priority / OrElse implicit search helper

Samples:

1. https://github.com/typelevel/algebra/blob/master/core/src/main/scala/algebra/Priority.scala
2. https://github.com/milessabin/shapeless/blob/master/core/src/main/scala/shapeless/orelse.scala

I've also encountered cases in which I wished to have this helper available with the Async / Concurrent type classes in Cats-Effect. So it would work like this:

1. if a Concurrent instance is in scope, build a cancelable task via Concurrent.cancelable
2. if only an Async instance is in scope, as a fallback, build an atomic task via Async.async

So this is a mechanism for optimizing the implementation in case a type class instance is available, or otherwise use the default. This optimization problem is also relevant for Haskell: https://stackoverflow.com/questions/44250854/use-specialized-implementation-if-a-class-instance-is-available

I think it's time we add something like this to cats-kernel.

Start by reading the linked Priority.scala and shapeless/orelse.scala examples, then compare the proposed Concurrent.cancelable and Async.async fallback behavior with the cats-kernel scope. Done would require an agreed API design for an implicit search helper and a clear decision on whether this belongs in cats-kernel.