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 Ptrs to return to the caller) and overlaps heavily with the iterator-based stack decoding implementation now provided by ghc-heap (see !9514 (closed)). We should move the stack decoding implementation from ghc-heap into ghc-internal and reimplement GHC.Stack.CloneStack.decode in terms of it.