Add Tween type.
Add Tween type.

Summary: Old Tween spec: https://material-motion.github.io/material-motion/starmap/specifications/plans/Tween

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

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

Tags: #material_motion

Differential Revision: http://codereview.cc/D2397

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