0.19.3: Add thru, Improve combine

• Add stream.thru(), which allows adapting functional APIs to fluent style
• Improve efficiency of combine()
• Add throttle fusion. For example: stream.throttle(n).throttle(m) becomes stream.throttle(Math.max(m, n))
• Experimental Add map/throttle commutation. For example, stream.map.throttle.map becomes stream.throttle.map.map, which is then fused into stream.throttle.map