Rename World to Entities

Entities seems the more fitting name, since managing entities is what it
does (and all it does). I originally imagined that more things would be
added (like support for systems, events), but my current thinking is
that this functionality belongs with other data types. We'll see, I can
always change it back.

I'm leaving the macro name as is for now. Right now, it's not very clear
to me what responsibilities it'll eventually have.
Make name of "Entities" struct configurable

I'm about to introduce a trait to be implemented by Entities and calling
both the same is bound to confuse people.

I'm not sure if making it fully configurable is the best solution and
I'm especially not sure about the syntax. Again, we'll see.
Rename world variables in the tests to "entities"
Rename add_entity to add

This is the first in a series of renames for the methods of Entities.
Now that it's called "Entities", it makes little sense to keep the
"_entity" suffix in all the method names.
Rename remove_entity to remove
Rename import_entity to import
Rename export_entities to export
Implement Entities trait for entities structure

I think this is already useful for some kinds of code (for example
generically importing entities that were received over the network). It
will also help with some other features I have in mind.