Extract the topological ordering into a module.

Multiple passes will need to iterate over EBBs in a
dominator-topological order. Move that functionality into a separate
module.

lib/cretonne/src/lib.rs

@@ -35,4 +35,5 @@ mod packed_option;
 mod partition_slice;
 mod predicates;
 mod ref_slice;
+mod topo_order;
 mod write;