`Layout` class not exposed

The `DragDropContext`'s `setOverlay()` action takes a `Layout` object. While building a custom `DragOverlay` replacement, I noticed that the `Layout` class from `layout.ts` is not exported by `index.ts`, complicating user-land `Layout` creation.

@martinpengellyphillips Would you accept a PR to expose the `Layout` class and, ideally, also the `elementLayout()` function?