Expose CFG predecessors only as an iterator.

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.

cranelift/src/print_cfg.rs

@@ -76,7 +76,7 @@ impl<'a> CFGPrinter<'a> {
 
     fn cfg_connections(&self, w: &mut Write) -> Result {
         for ebb in &self.func.layout {
-            for &(parent, inst) in self.cfg.get_predecessors(ebb) {
+            for (parent, inst) in self.cfg.pred_iter(ebb) {
                 writeln!(w, "    {}:{} -> {}", parent, inst, ebb)?;
             }
         }