This change was made to make it easier for invariants to be upheld. When
the spatial partition is added, we can ensure that changes to entities
are immediately reflected in the partition. Additionally, chunks being
shared between worlds was a leaky abstraction to begin with and is now
removed. A method in `Config` is now necessary to determine what world a
client should join.
Along with this, most mutable references have been wrapped in a newtype
to ensure that `mem::swap` cannot be used on them, which would break
invariants. This is analogous to `Pin<&mut T>`. The reason we can't use
Pin directly is because it would require unnecessary unsafe code
within the library.
This lets us use slotmap keys between different instances of the slotmap
without unspecified behavior happening. Also, we no longer have to worry
about version overflow. The downside is that the keys are now 12 bytes
instead of 8, but this is an acceptable trade if it means fewer bugs.