Resume on resumable session invalidations

Session invalidations include whether the session may be resumable.
Previously, this was not parsed by serenity. This data is now contained
in `GatewayEvent::InvalidateSession` as a boolean, which constitutes a
breaking change for users that manually handle gateway events.

Upgrade path:

Instead of pattern matching on simply the variant like so:

```rust
use serenity::model::event::GatewayEvent;

match gateway_event {
    GatewayEvent::InvalidateSession => {
        // work here
    },
    // other matching arms here
    _ => {},
}
```

Instead pattern match it as a single field tuple-struct:

```rust
use serenity::model::event::GatewayEvent;

match gateway_event {
    GatewayEvent::InvalidateSession(resumable) => {
        // work here
    },
    // other matching arms here
    _ => {},
}
```

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