Add states stack

[voxelias]

This PR implements application states stack.

Developer can define rules on what states what systems should run. By default any system runs on any state.

States are identified by data types and can be structure with or without data:

struct MainState {
    name: String,
}

struct PauseState {
    name: String,
    handled: bool,
}

This will add rule to enable system only when MainState is active

.with(
    System::from(camera::control)
        .with(State::on::<MainState>())
)

There could be more rules in fact:

.with(
    System::from(player::control)
        .with(State::on::<MainState>())
        .with(State::on::<InventoryState>())
)

It is also possible to enable on all but some states:

.with(
    System::from(ui_main)
        .with(State::off::<PauseState>())
)

There is a new service State that implements API to control the state of application:

fn startup(
    mut state: Mut<State>,
) {
    // Push main state
    state.push(MainState {
        name: String::from("Main state")
    });
}

When you need, you can return to previous state:

fn system_where_you_pop_the_state (
    mut state: Mut<State>,
) {
   ....
    if exit_state {
        state.pop_any();
    }
}