unique_components allows read access to !Sync resources from any thread

[james7132]

Archetype::unique_components allows access to read any !Sync resource from any arbitrary thread. This allows for aliasing accesses to these resources given a &World. This is unsound.

[james7132]

After #4809, this seems to be mitigated from a user-facing perspective, but otherwise remains untouched from an internal perspective.