Improve '<~' (left-closure reacting operator) by returning same Stream.
Improve '<~' (left-closure reacting operator) by returning same Stream.

slxl/ReactKit,ReactKit/ReactKit,wantedly/ReactKit,morizotter/ReactKit,wantedly/ReactKit,ReactKit/ReactKit,slxl/ReactKit,morizotter/ReactKit,wantedly/ReactKit,morizotter/ReactKit