Make everything a slice

[cowboyd]

depends on #559

Right now we have a lot of duplication in the Atom code and the Slice code. because in addition to housing the value, the atom can be sliced and diced directly. Instead, let's refactor atom to be just a simple referefence to a subscription of values with set() method that allows you to publish a new value.

Then, Slice can just take an atom in its constructor, and everything in the app becomes a slice. We could even have a createAtom function, return a (root) slice:

declare function createAtom<S>(initial: S): Slice<S>