Make it possible to ask for a *[]T

If you put pointer: true on a list-field, you get []*T (or [][]*T, or...) which is probably what you wanted (so you can represent an required list of optional items, for example, but not the only option. If anyone finds a reason to want *[]T, or for that matter *[][]*[]T, we can figure out a syntax for it; it's easy enough to do technically. (Although it won't be totally trivial if T is an interface.)