add Clone on triggers if available

[hyranno]

This PR add Clone and some common traits onto triggers and some values for convenience. It is good for reusing triggers.

Implementing Clone on StateMachine will be convenient too, but it will conflict with pending PR (#7 ), so I won't implement until it is closed (weather merged or not).

[Seldom-SE]

StateMachine used to implement Clone, but I removed it so that triggers don't have to implement Clone. I recommend this pattern:

let state_machine_builder = || {
    StateMachine::default()
        .trans<MyState>(MyTrigger, MyOtherState)
};

commands.spawn((
    MyState,
    state_machine_builder(),
));

commands.spawn((
    MyOtherState,
    state_machine_builder(),
));

Or break it out into a separate function. I'd accept the impls that are here, though. But I'll wait to see what you think first.

[hyranno]

Sounds reasonable. It is good to allowing things to be not Clone, and it looks hard to implement Clone for StateMachine without forcing its member to be Clone. I expected these merits to implement Clone for StateMachine:

• Clone will be faster than repeated initializer calls.
• It will be easier to implement Clone for impl Bundle which contains StateMachine.

But I think these merits are less important.

[Seldom-SE]

Thanks!