Breaking changes:

+ `Lens` module no longer nested inside `Focus` module
+ `view/2`, `over/3`, and `set/3` are now the core of the `Focusable`
protocol. `Lens` implements `Focusable` (as does the new `Prism`
module).
+ `Lens.view/2`, `Lens.over/3`, and `Lens.set/3` are now accessible
as `Focus.view/2`, `Focus.over/3`, and `Focus.set/3`

The other changes are primarily adding new functionality to lenses and
adding prisms.