Bind reading functions to the lens

[couzic]

It would allow more concise syntax. For example:

const state$: Observable<State> = ...
const lens = createLens<State>

const a$: Observable<A> = state$.map(state => lens.focusOn('a').read(state))

The last line could be written:

const a$: Observable<A> = state$.map(lens.focusOn('a').read)

[couzic]

NO GO. It would add a performance overhead for each created Lens. Problem with be solved with more API design.