Add log operator.
Add log operator.

Summary: Spec: https://material-motion.github.io/material-motion/starmap/specifications/streams/operators/debugging/$.log

Reviewers: markwei, #material_motion, O4 Material Apple platform reviewers, O2 Material Motion

Reviewed By: markwei, #material_motion, O4 Material Apple platform reviewers, O2 Material Motion

Tags: #material_motion

Differential Revision: http://codereview.cc/D2435

material-motion/material-motion-swift,material-motion/material-motion-swift