Reorder update fn signature to State, Action, Environment

[gordonbrander]

This PR changes the order of update function arguments:

• Changes update function signature from update(State, Environment, Action) to update(State, Action, Environment)
• Changes Store from Store<State, Environment, Action> to Store<State, Action, Environment>

Why?

• This order of arguments is used by two other Elm-like libraries: Swift Composable Architecture and Redux-like state container.
• Context arguments like environment often seem to be passed last in Swift. E.g. UIViewRepresentable.Context is passed last to methods in SwiftUI.
• State and Action are always used, but Environment might be nil, or an empty struct. You don't always need environment. It is optional.
• The order reads better in the signature, since state and action are always paired together in update function and Update struct.