Fake World::modify impl

[Ralith]

Proof-of-concept for #19. Currently blocked by an ICE in the doctest:

error: internal compiler error: src/librustc/traits/codegen/mod.rs:55: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@src/world.rs:10:5: 10:26] as std::ops::FnMut<(hecs::Entity, <hecs::query::FetchRead<bool> as hecs::Fetch<'_>>::Item)>>), Binder(<[closure@src/world.rs:10:5: 10:26] as std::ops::FnMut<(hecs::Entity, &bool)>>), Sorts(ExpectedFound { expected: &bool, found: <hecs::query::FetchRead<bool> as hecs::Fetch<'_>>::Item }))` selecting `Binder(<[closure@src/world.rs:10:5: 10:26] as std::ops::FnMut<(hecs::Entity, &bool)>>)` during codegen

[Astralchroma]

The internal compiler error seems to be resolved now, couldn't this potentially be revisited now?

[Ralith]

Probably! I forget if there was a realistic plan to make this work any better than using a command buffer, though. It's a pretty un-ergonomic API.