! Restructure completely

* Renamed FillableViewable to Listable
* Listable no longer inherits Viewable due to different variance of W
* Added PartialViewable, PartialListable with orElse combinators
* Renamed FillableViewableAdapter to ListableAdapter
* Renamed SlottedFillableViewable to SlottedListable (aliased to Listable.Slotted)
* Removed TweakFillableViewable, TransformerFillableViewable
* Moved Ops to the package object