Split buttons and axes and improve ergonomics of working with gamepad input

[alice-i-cecile]

Objective

• Principally fixes Button data is typically binary, but is treated as if it were continuous #3398.
• Incidentally fixes Type-level disparity between keyboard and gamepad input APIs #3224.
• Significantly improves the ergonomics of working with gamepad axes and buttons, as you no longer need to constantly generate GamepadButton structs which store both the gamepad id and the button that is being pressed.

Solution

1. Adds a value and set_value method to Input, which is appropriately updated. 0.0 is unpressed, 1.0 is pressed, following industry convention.
2. Made it impossible to use a button-like type (0.0 to 1.0) as an axis (-1.0 to 1.0) by adding the methodless Inputlike and Axislike traits.
3. Incidentally made it possible to iterate over all of the possible variants of the input types, by pulling in the strum crate's EnumIter trait. We already had some constants for this, which have been removed.

This was simple enough, except that I could not accomplish this nicely for GamepadButton, as its variant type and the input type were not the same (see #3224). Instead of adding an ugly hack with an associated type that was almost always Self I:

1. Eliminated the existing GamepadButton type, and gave the nice name to GamepadButtonType.
2. Changed the appropriate Input type to accept the enum.
3. Managed the "you can have multiple controllers" problem by handing out multiple Input<GamepadButton> types using the freshly added Gamepads type (Make GamepadLobby a built-in type #3245), rather than a single resource which required you to pass in the Gamepad constantly.
4. Eliminated the dedicated Gamepads management system by rolling it all into the existing gamepad_event_system.
5. Did the same for GamepadAxis and GamepadAxisType for consistency and a wonderfully smooth API.

Showcase

Before:

fn gamepad_system(
    gamepads: Res<Gamepads>,
    button_inputs: Res<Input<GamepadButton>>,
    button_axes: Res<Axis<GamepadButton>>,
    axes: Res<Axis<GamepadAxis>>,
) {
    for gamepad in gamepads.iter().cloned() {
        if button_inputs.just_pressed(GamepadButton(gamepad, GamepadButtonType::South)) {
            info!("{:?} just pressed South", gamepad);
        } else if button_inputs.just_released(GamepadButton(gamepad, GamepadButtonType::South)) {
            info!("{:?} just released South", gamepad);
        }

        let right_trigger = button_axes
            .get(GamepadButton(gamepad, GamepadButtonType::RightTrigger2))
            .unwrap();
        if right_trigger.abs() > 0.01 {
            info!("{:?} RightTrigger2 value is {}", gamepad, right_trigger2);
        }

        let left_stick_x = axes
            .get(GamepadAxis(gamepad, GamepadAxisType::LeftStickX))
            .unwrap();
        if left_stick_x.abs() > 0.01 {
            info!("{:?} LeftStickX value is {}", gamepad, left_stick_x);
        }
    }

After:

fn gamepad_system(gamepads: Res<Gamepads>) {
    for (gamepad, buttons) in gamepads.buttons.iter() {
        if buttons.just_pressed(GamepadButton::South) {
            info!("{:?} just pressed South", gamepad);
        } else if buttons.just_released(GamepadButton::South) {
            info!("{:?} just released South", gamepad);
        }

        let right_trigger2 = buttons.value(GamepadButton::RightTrigger2);
        if right_trigger2 > 0.01 {
            info!("{:?} RightTrigger2 value is {}", gamepad, right_trigger);
        }
    }

    for (gamepad, axes) in gamepads.axes.iter() {
        let left_stick_x = axes.get(GamepadAxis::LeftStickX).unwrap();
        if left_stick_x.abs() > 0.01 {
            info!("{:?} LeftStickX value is {}", gamepad, left_stick_x);
        }
    }
}

[alice-i-cecile]

bors try

[bors]

try

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