Store action-specific state in `ActionState`

[alice-i-cecile]

Many actions are only meaningful in association with non-enumerable data.

A very useful pattern here is to store information in the enum variant, and then update that appropriately.

We want to be able to express:

#[derive(Actionlike, Clone)]
enum ArpgAction {
    Movement(Direction),
    Ability1,
    Ability2,
    Ability3,
    Ability4,
    Ultimate,
}

input_map.insert(ArpgAction::Movement(Direction::NORTH), KeyCode::Up);

let ArpgAction::Movement(direction) = action_state.get_value::<ArpgAction::Movement>();

Unfortunately, a reasonable API for the last step seems to be completely blocked on rust-lang/rfcs#2593, which, unsurprisingly, is postponed indefinitely 🙃

[alice-i-cecile]

Blocks #50.

[alice-i-cecile]

We can eliminate the Hash bound by storing a HashMap<usize, InputAction> in InputMap, and doing likewise for ActionMap. This usize is given by Actionlike::index(), which has the desired behavior!

This may make #68 harder, but this feature is significantly more important, and serves related goals.

[alice-i-cecile]

We may be able to workaround this by generating an internal type to match those of each variant, and then requesting those in ActionState::get_value :/ It's extremely cursed though due to the high levels of magic, and will do wonky things with the imports.

[alice-i-cecile]

This is fundamentally wrong-headed, and not our responsibility.