Ability to create a query view directly from `World`

[LPGhatguy]

hecs has a great mechanism for random access of a query via views:

let mut query = game.world.query::<(&A, &B, &C)>();
let view = query.view();
let (a, b, c) = view.get(some_entity)?;

It'd be really handy to collapse the calls to query and view into just one call, to avoid needing to create the extra binding.

I think there are a couple good solutions to this problem:

• A new method, World::view and a new View type that owns its contents
• Refactoring of QueryBorrow to enable random access

[Ralith]

Refactoring of QueryBorrow to enable random access

This would involve taking the fetch field of View, which is a table with one entry per archetype, and adding it to query, populating it either up front (which adds unnecessary work to normal queries) or lazily (which adds a branch to every random access, the case that views are specifically supposed to optimize). Given those (minor) drawbacks, I think I lean slightly towards paying the modest one-time cost of introducing a new type (perhaps ViewBorrow).

Are you interested in making a PR for this?

[LPGhatguy]

I'd be interested in working on this feature, but I'm pretty booked for a couple months. If no one else comes along and takes it by then, I can take a look!

[LPGhatguy]

I created a cursed and unsafe version of this API in the hecs wrapper we have in our engine. We use views a lot, so I think it will be very nice to have.

I'm going to use it for awhile and see whether it's a very large gain. If it is, I'll take a go at implementing it properly in hecs!

[Ralith]

This would incrementally tidy up some of the code I've been writing lately too.

[Ralith]

Fixed by #367.