rts: retainer: Generalise per-stackElement data
This essentially ammounts to s/retainer/stackData/, s/c_child_r/data/ and some temporary casting of c_child_r to stackData until refactoring of this module is completed by a subsequent commit. We also introduce a new union 'stackData' which will contain the actual extra data to be stored on the stack. The idea is to make the heap traversal logic of the retainer profiler ready for extraction into it's own module. So talking about "retainers" there doesn't really make sense anymore. Essentially the "retainers" we store in the stack are just data associated with the push()ed closures which we return when pop()ing it.
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