Add iterators for bforest::{Set,Map}.

The iter() methods return an iterator that traverses all set elements /
map key-value pairs. The iterator doesn't require a mutable container
and forest reference, unlike the cursor types.

lib/cretonne/src/bforest/mod.rs

@@ -22,8 +22,8 @@ mod path;
 mod pool;
 mod set;
 
-pub use self::map::{MapForest, Map, MapCursor};
-pub use self::set::{SetForest, Set, SetCursor};
+pub use self::map::{MapForest, Map, MapCursor, MapIter};
+pub use self::set::{SetForest, Set, SetCursor, SetIter};
 
 use self::node::NodeData;
 use self::path::Path;