Make PointerType work like RecursiveType

[calebsander]

Still need a separate type because RecursiveType only reuses objects that are strictly equal, while pointers should reuse anything with the same value representation.

• If first instance of value (as judged by value buffer), write value
• Otherwise write a flexInt offset to it
• There should no longer be any need for placeholder pointer bytes and having the root type call setPointers()