The generic implementation was calling the non-generic one and then using an additional continuation to extract the resulting `Task<TResult>` due to lack of covariance on classes.  We can instead just factor the shared implementation out into a generic with the type parameter constrained to be a Task.  This results in simpler code as well as avoiding an extra continuation in the generic case.

As part of cleaning this up:
- I changed code where we need to make a defensive copy of an input collection to use CopyTo; we were already doing this in some places but not others.  This saves on an enumerator allocation when enumerating the source collection, as well as multiple interface calls.
- I augmented WhenAny to also special-case `List<Task>`, as that's a common input and we can handle it a bit more efficiently, especially if the collection ends up containing just two tasks.
- I removed the `GenericDelegateCache<TAntecedentResult, TResult>`.  That was from a time before the C# compiler supported caching of generic lambdas. It would have needed to have been updated to handle the stronger type coming out of CommonCWAnyLogic, so I instead just got rid of it.  We're better off lazily-creating these rarely used delegates, anyway.