Allow for querying a specific variant of an Enum+Component

## What problem does this solve or what need does it fill?
Imagine I have a component which is an enum , like AbilityState

```
#[derive(Component)]
pub enum AbilityState {
Preparing,
Casting,
Complete
}

```

It would be very nice to have a way to query for ONLY the entities who have that component AND in which the component is set to a particular variant.

The way it is now, I have to query for all entities with that component and then match on the state which is fine but quite verbose and silly looking since i have 1 system per variant.