Remove `SystemBundle`s and add `System`s directly

[azriel91]

In GitLab by @azriel91 on Oct 31, 2019, 15:57

Bundles currently provide two functionalities:

• Group systems, and add them together.
• Add resources to the World.

However, the use of bundles also makes it difficult to specify system dependencies, as the bundle would need to take in / output the names of the systems. As such, most bundles do not do this, and so most systems are run in any order.

We may use bundles for adding resources, but we should look at adding Systems to the dispatcher (via GameDataBuilder) without SystemBundles.

If Amethyst switches to Legion, we may use that migration opportunity to also do this.

Related: #138