Structure exports

[appsforartists]

The root namespace of material-motion is littered with Motion____able. Perhaps we should organize the exports:

import {
  operatorInterfaces,
} from 'material-motion';

const {
  MotionPluckable,
} = operatorInterfaces;

The syntax is a bit gross, but they are seldomly useful to 3rd parties, so probably shouldn't pollute the exported namespace.