Make Point a consistent type

In the spec we have many occurrences of the `Point` type. Sometimes it's a class; sometimes it's a struct. Sometimes the fields are private; sometimes they are public.

We should be as consistent as possible - it may be okay for it to vary a little bit in implementation over the course of the spec, but only to demonstrate specific points.

I'll go over the spec and try to categorize how it's being used in different places.