Test the return type of 'unaryFun' and 'binaryFun'.

[Mek101]

I made the following enumerations to test the predicates.

>private enum bool isUnaryPredicate(alias pred, Range) = 
>is(typeof(unaryFun!(pred)((ElementType!Range).init)) == bool);
>private enum bool isBinaryPredicate(alias pred, Range, V) = 
>is(typeof(binaryFun!(pred)((ElementType!Range).init, V.init)) == 
>bool);

Both work as expected. Thank you.