The previous GC implementation used a recursive mark method. This can
result in stack overflows when attempting to mark deeply nested objects.
This commit replaces the recursive approach with an iteritive one,
moving the state stack from the C call stack to the `WrenVM` structure.
As objects are 'grayed' they are pushed onto the VM's gray stack. When
we have grayed all of the root objects we iterate until the stack is
empty graying any obejcts which haven't been marked as dark before. At
the end of the process we clean up all unmarked objects as before.
This commit also adds a few new tests which check garbage collection by
allocating some new deeply nested objects and triggering the GC a few
times in the process.
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