Mark and Sweep

Mark and Sweep garbage collection was first described by John McCarthy in 1960, primarily for managing memory in ((lisp)). It's a two-phase algorithm:

1. Mark Phase: Traverses the object graph, marking all reachable objects.
2. Sweep Phase: Scan memory, collecting all unmarked objects, which are considered garbage.

Note! We don't keep track of how many times a particular object is referenced, like we did with reference counting! Instead, we keep track of which objects are referenced in each stack frame and then traverse our container objects looking for any other referenced objects. That's what "traverse the object graph" means – a fancy way of saying "look for objects".

1. In snekobject.h add an is_marked boolean to our snek_object_t struct.
2. In sneknew.c ensure the is_marked field is set to false when we create a _new_snek_object.