Add `world` and `world_mut` methods to `WorldChildBuilder`

[benfrankel]

What problem does this solve or what need does it fill?

Allow World access when you only have a WorldChildBuilder.

What solution would you like?

Add world and world_mut methods to WorldChildBuilder, returning references to the inner &mut World. This doesn't have to be unsafe like EntityWorldMut::world_mut, because the latter is only unsafe due to the EntityLocation stored in EntityWorldMut that could become invalidated. There is no EntityLocation stored in WorldChildBuilder.

What alternative(s) have you considered?

None.

[benfrankel]

It may be nice to add ChildBuilder::commands as well. Currently there's only a queue_command method there, which is insufficient if you want to access e.g. an extension trait or method on Commands.

[viridia]

Just FYI, My main reason for wanting this is to add extension traits on WorldChildBuilder.