Add iteration support to SparseMap.
Add iteration support to SparseMap.

This is simply the slice iterator for the dense vector.

- map.values() returns an iterator with references to the values.
- for i in &map iterates over references to the values.

stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne