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Eliminate stg_decodeStackzh
`GHC.Stack.CloneStack.decode` is currently implemented in terms of `stg_decodeStackzh` (defined in `rts/CloneStack.c`). However, this implementation is rather inefficient (allocating an array of boxed `Ptr`s to return to the caller) and overlaps heavily with the iterator-based stack decoding implementation now provided by `ghc-heap` (see !9514). We should move the stack decoding implementation from `ghc-heap` into `ghc-internal` and reimplement `GHC.Stack.CloneStack.decode` in terms of it.
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