Require `$id`

[gregsdennis]

All schemas should have an identifier.

[jdesrosiers]

I disagree with this one. I know the spec recommends that schemas include an identifier, but I've come to believe that's bad advice. I find it's better to use natural identifiers such as file paths or network locations. If a schema is truly in-memory-only, you can always provide an identifier when loading a schema rather than use $id.