Derive Serialize/Deserialize for VirtualKeyCode

[kerskuchen]

It would be useful for VirtualKeyCode (and possibly other structs in event.rs) to be serializable/deserializable to write and read in custom key-mapping files that can be consumed directly by applications.

For example this would allow for serialized HashMaps like

(
    key_mapping: {
        Up: ["ui_up","move_up",],
        Down: ["ui_down","move_down",],
        Left: ["ui_left","move_left",],
        Right: ["ui_right","move_right",],
    },
)

be used by a winit-agnostic process_buttons function like

match event {
    WindowEvent::KeyboardInput {
        input:
            KeyboardInput {
                state: glutin::ElementState::Pressed,
                virtual_keycode: Some(key),
                // modifiers,
                ..
            },
        ..
    } => match key {
        let buttons = key_mapping[key];
        process_buttons(&buttons);
    }
}

without needing to change the event loop.

[kerskuchen]

I just realized this is a duplicate of #304