Refactoring: GC

[m4drat]

1. Make GC pass-based (do we need it?)

Future work - explore some different approaches: "So the solution Azul proposes goes like this: They establish a "read barrier" that allows the GC to intercept dereferencing, and this way they can lazily update the references that are actually used.". Maybe we can update references lazily (as we always have some kind of a pointer abstraction)