add state.StateTree#Version() accessor.

chain/state/statetree.go

@@ -395,6 +395,11 @@ func (st *StateTree) ForEach(f func(address.Address, *types.Actor) error) error
 	})
 }
 
+// Version returns the version of the StateTree data structure in use.
+func (st *StateTree) Version() builtin.Version {
+	return st.version
+}
+
 func Diff(oldTree, newTree *StateTree) (map[string]types.Actor, error) {
 	out := map[string]types.Actor{}