Define two public iterator types in the flowgraph module, PredIter and
SuccIter, which are by-value iterators over an EBB's predecessors and
successors respectively.
Provide matching pred_iter() and succ_iter() methods for inspecting the
CFG. Remove the get_predecessors() method which returned a slice.
Update the uses of get_predecessors(), none of which depended on it
being a slice.
This abstraction makes it possible to change the internal representation
of the CFG.
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