Implement Extend for World

[Anders429]

It may be possible to generically implement Extend for World in a generic way that allows for extending from any iterator that yields values implementing Entity. If this is found to be possible, IntoIterator could also be implemented for Batch<Entities> (which still needs to be renamed, by the way), allowing extend to work with that struct.

One main issue is that our current extend method returns EntityIdentifiers corresponding to the inserted values. This would not be possible with the Extend trait. At the very least, however, Batch<Entities> should be made to implement IntoIterator, and also should work with inputs besides Vec.

[Anders429]

This is basically blocked on specialization. The pull request I put together (#96) is getting stale as the API expands and grows in other ways, and as it currently stands, that PR isn't going to be merged anyway due to the performance regression present.

[Anders429]

This may be something worth addressing again. I've recently been working on following the rustrogueliketutorial, using brood in place of specs wherever necessary. This has been enlightening in many ways, as it leads me to see weaknesses in the API and feature set from a user perspective.

One of the things I have seen is that using World::extend() is currently very limiting. Either you have a static set of component columns, or you don't, and that ends up being rather inflexible.

It is likely worth looking at this again to see if there is any way to make this more flexible.