Adding and removing multiple components from an entity.

[Anders429]

Currently, Entry::add() and Entry::remove() only support removing one component at a time. This means that the entity will have to be moved to a new archetype with every change if multiple changes are desired.

To account for this, Entry::add() and Entry::remove() should be modified to take multiple components. Additionally, a new method called Entry::add_remove() could be introduced which both adds and removes components at the same time.

[Anders429]

I've thought about this for a bit now, and I think the best way to implement this would be to also introduce a Components trait, along with components!() and Components!() macros. This makes more sense semantically than reusing the existing entity!() and Entity!() macros, since we're not directly creating an entity, just a set of components.

On that same note, it would probably make sense to combine these in the future. Technically, when a new entity is added to a World with World::insert(), it isn't really an "entity" before it is actually added, at which point it is given an entity::Identifier. Just defining something with entity!() doesn't actually make it an entity. It may make sense to combine entity!() and entities!() into this new components!() macro (and same for the type macros). This is work that can be done in the future, and won't be tracked here.