Tag component

[leudz]

It's possible to add empty components but they'll still use an EntityId per component and up to an usize per entity in the World (could go down with #10 if the tagged entities are in the right spot).
This could be far less, one bit per entity in the World with the possibility to go even lower using pagination.

Since the storage wouldn't store EntityId it won't be possible to call with_id on these storages but with #52 we'll still be able to iterate them.
There would be a new Tag type used like Unique to access the storage.