Structure exports

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