Allow client to specify their own state shape

[effa]

How should be the state shape specified?

Related question: How to make initial states and reducers be orthogonal (because we will want to start in different initial states all the time, consider web - for each request new initial state is created), but in sync (= both should be validated against (or depend on?) a common state shape contract)?

Currently the state shape is implicitly defined on more places: reducers.py and state.py. Should be on a single place. Related problem with the current version is that after a reduction, state shape can be changed by mistake (by a typo in reducers.py)

[effa]

State should check that all keys are valid (at least on initialization?) Why: to prevent typos-errors when creating own initial state.