Add `Input` convenience resource

[cart]

Currently consuming input events looks like this:

fn move_on_input(
    world: &mut SubWorld,
    mut state: ResMut<State>,
    time: Res<Time>,
    keyboard_input_events: Res<Events<KeyboardInput>>,
    query: &mut Query<(Write<Translation>, Read<Handle<Mesh>>)>,
) {
    let mut moving_left = false;
    let mut moving_right = false;
    for event in state.event_reader.iter(&keyboard_input_events) {
        if let KeyboardInput {
            virtual_key_code: Some(key_code),
            state,
            ..
        } = event
        {
            if *key_code == VirtualKeyCode::Left {
                moving_left = state.is_pressed();
            } else if *key_code == VirtualKeyCode::Right {
                moving_right = state.is_pressed();
            }
        }
    }

    for (mut translation, _) in query.iter_mut(world) {
        if moving_left {
            translation.0 += math::vec3(1.0, 0.0, 0.0) * time.delta_seconds;
        }

        if moving_right {
            translation.0 += math::vec3(-1.0, 0.0, 0.0) * time.delta_seconds;
        }
    }
}

It would be great if it looked like this instead.

fn move_on_input(
    world: &mut SubWorld,
    mut state: ResMut<State>,
    time: Res<Time>,
    input: Res<Input>,
    query: &mut Query<(Write<Translation>, Read<Handle<Mesh>>)>,
) {
    let mut moving_left = input.key_pressed(VirtualKeyCode::Left);
    let mut moving_right = input.key_pressed(VirtualKeyCode::Right);

    for (mut translation, _) in query.iter_mut(world) {
        if moving_left {
            translation.0 += math::vec3(1.0, 0.0, 0.0) * time.delta_seconds;
        }

        if moving_right {
            translation.0 += math::vec3(-1.0, 0.0, 0.0) * time.delta_seconds;
        }
    }
}

The raw events are a good first step (and shouldnt be removed), but the common "was this input pressed before this update" case should be optimized.