Add a dropReturn MethodHandle combinator

Add a method to MethodHandles, like:

    public static MethodHandle dropReturn(MethodHandle target)

That can be used to drop the return value of a MethodHandle.

Doing this is currently already possible with the following code:

        MethodHandle target = ...;
        Class<?> targetReturnType = target.type().returnType();
        if (targetReturnType == void.class)
            return target;
        return filterReturnValue(target, empty(methodType(void.class, targetReturnType)));

But, a combinator can be provided as a short-hand.