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rts: retainer: Turn global traversal state into a struct

Global state is ugly and hard to test. Since the profiling code isn't quite
as performance critical as, say, GC we should prefer better code here.

I would like to move the 'flip' bit into the struct too but that's
complicated by the fact that the defines which use it directly are also
called from ProfHeap where the traversalState is not easily
available. Maybe in a future commit.