Refactor access patterns of `UnsafeWorldCell`

[alice-i-cecile]

Not super comfortable with this, since the rest of UnsafeWorldCell's method don't use or respect this memory access pattern (world_mut is a special case). They just create and encapsulate &World references and provide restricted access. It's fine for now though since it's private and unsafe, but I think we should refactor the rest of UnsafeWorldCell to use this memory access pattern, and probably remove world_metadata since it's possible to access metadata safely through provided methods.

Originally posted by @JoJoJet in #10756 (comment)