Demonstrate how to validate manifests

[alice-i-cecile]

What problem did the developer encounter?

Ensuring that manifests are valid is an incredibly important quality control step.

Doing so is currently left to the user's imagination however.
This is particularly important when dependent manifests are used: dead links are common and will cause subtle failures.

Types help a lot, but other errors could exist!

How could this be better documented?

1. Showcase how to load all manifests in a test, and assert that they load correctly.
2. Demonstrate how to perform bounds-checking on field values.
3. Demonstrate how to validate that all IDs to entries in other manifests are valid.
4. Demonstrate how to check that all assets are used by an entry in a manifest.
5. Demonstrate how to check that all other required assets can be loaded.