add ToDo and setup.rb

Author: masa16

ToDo

@@ -0,0 +1,11 @@
+
+* reduce loop: copy to buffer if non-contiguous.
+* unify contiguous dimesions.
+
+* inspect Enumerable#step object, delete Numo::Step class.
+
+* move "reduce" field from RNArray to RNArrayView.
+
+* unify RNArray structure??
+
+* how to get the element size value effectively??