Distinct IContinuation types/parameter names

[smagi]

Need to add runtime checks to ensure that the type parameters to IContinuation are:

1. all distinct types, and
2. not T at one arg, and IEnumerable<T> at another arg

It would be nice if at least 1 could be enforced statically, but I don't see a way to do it. IContinuationX could implement a basic IParam<T> interface, but if you try to implement it for multiple type parameters, you get an error:

// error: 'IContinuation<T0,T1>' cannot implement both 'IParam<T0>' and 'IParam<T1>' because they may unify
// for some type parameter substitutions
public interface IContinuation<T0, T1> : IParam<T0>, IParam<T1> { }

Basically, this is only an error when defined for the same type parameters at the *use* site, not at the *declaration* site, but C# doesn't defer this check as I think it should.

This still wouldn't help for case 2 though, so we need a runtime check regardless. Fortunately, this can be checked once when the type is first loaded as a continuation.