feat: make World only ever require &self. (#270)

This is wide but relatively shallow update that makes `World` only ever
require `&self`, allowing us to remove the system initialization stage
that was necessary to have `&mut World` access for initializing
resources and components.

The main change was to allow `Resources` to have resources added to it
with only a `&self` reference by using a `once_map` internally. This
meant that all resource cells now become `Option<SchemaBox>` on the
inside. This conceptually means that cells for reasources are lazily
initialized and always considered to be present, but they may or may not
actually contain a resource value.

demos/features/src/main.rs

@@ -274,11 +274,7 @@ fn menu_system(
                     //
                     // This makes it easier to compose widgets that have differing access to the
                     // bones world.
-                    //
-                    // Note that all of the parameters of the system must have been initialized
-                    // already, so if they are not initialized by another system, you have to use
-                    // `world.init_param::<SomeParam>()` in the session plugin to initialize them.
-                    world.run_initialized_system(demo_widget, ui);
+                    world.run_system(demo_widget, ui);
 
                     ui.add_space(30.0);
                 });

framework_crates/bones_bevy_renderer/src/lib.rs

@@ -621,7 +621,7 @@ fn load_egui_textures(
         // TODO: Avoid doing this every frame when there have been no assets loaded.
         // We should should be able to use the asset load progress event listener to detect newly
         // loaded assets that will need to be handled.
-        let mut bones_egui_textures = bones_egui_textures_cell.borrow_mut();
+        let mut bones_egui_textures = bones_egui_textures_cell.borrow_mut().unwrap();
         // Take all loaded image assets and conver them to external images that reference bevy handles
         bones_image_ids.load_bones_images(
             asset_server,
@@ -665,7 +665,7 @@ fn egui_input_hook(
     mut data: ResMut<BonesData>,
 ) {
     if let Some(hook) = data.game.shared_resource_cell::<bones::EguiInputHook>() {
-        let hook = hook.borrow();
+        let hook = hook.borrow().unwrap();
         let mut egui_input = egui_query.get_single_mut().unwrap();
         (hook.0)(&mut data.game, &mut egui_input);
     }
@@ -782,7 +782,7 @@ fn step_bones_game(world: &mut World) {
             data.game.shared_resource_cell().unwrap()
         }
     };
-    let bones_window = bones_window.borrow_mut();
+    let bones_window = bones_window.borrow_mut().unwrap();
 
     let is_fullscreen = matches!(&window.mode, WindowMode::Fullscreen);
     if is_fullscreen != bones_window.fullscreen {

framework_crates/bones_ecs/examples/pos_vel.rs

@@ -31,9 +31,6 @@ fn main() {
         .add_system_to_stage(CoreStage::Update, pos_vel_system)
         .add_system_to_stage(CoreStage::PostUpdate, print_system);
 
-    // Initialize our systems ( must be called once before calling stages.run() )
-    stages.initialize_systems(&mut world);
-
     // Run our game loop for 10 frames
     for _ in 0..10 {
         stages.run(&mut world);

framework_crates/bones_ecs/src/lib.rs

@@ -77,7 +77,7 @@ mod test {
 
     #[test]
     fn insert_comp_with_gap() {
-        let mut w = World::new();
+        let w = World::new();
 
         #[derive(HasSchema, Default, Clone)]
         #[repr(C)]