refactor(engine): use Rust edition 2021

This change is because of a rustfmt bug in the 2024 edition.

For example, the following code snippet:
```
struct Value {}
struct ParsingError {}
trait Keyword {}

impl Value {
    fn parse<KeywordT: Keyword>(value: &str, line_no: usize) -> Result<Self, ParsingError> {
        todo!();
    }
}
```

when formatted with `rustfmt --edition 2021 --emit stdout --config max_width=90,brace_style=AlwaysNextLine` becomes:
```
struct Value {}
struct ParsingError {}
trait Keyword {}

impl Value
{
    fn parse<KeywordT: Keyword>(value: &str, line_no: usize)
    -> Result<Self, ParsingError>
    {
        todo!();
    }
}
```

1 files changed, 8 insertions, 7 deletions

diff --git a/engine/src/shader.rs b/engine/src/shader.rs
index d5611ca..7988724 100644
--- a/engine/src/shader.rs
+++ b/engine/src/shader.rs
@@ -6,10 +6,6 @@ use std::path::Path;
 use std::str::Utf8Error;
 
 use bitflags::{bitflags, bitflags_match};
-use crate::ecs::pair::{ChildOf, Pair};
-use crate::ecs::phase::{POST_UPDATE as POST_UPDATE_PHASE, Phase, START as START_PHASE};
-use crate::ecs::sole::Single;
-use crate::ecs::{Component, Sole, declare_entity};
 use shader_slang::{
     Blob as SlangBlob,
     ComponentType as SlangComponentType,
@@ -32,6 +28,10 @@ use crate::asset::{
     Submitter as AssetSubmitter,
 };
 use crate::builder;
+use crate::ecs::pair::{ChildOf, Pair};
+use crate::ecs::phase::{POST_UPDATE as POST_UPDATE_PHASE, Phase, START as START_PHASE};
+use crate::ecs::sole::Single;
+use crate::ecs::{Component, Sole, declare_entity};
 use crate::shader::default::{
     ASSET_LABEL,
     enqueue_set_shader_bindings as default_shader_enqueue_set_shader_bindings,
@@ -77,7 +77,7 @@ pub struct Module
 
 impl Module
 {
-    pub fn entry_points(&self) -> impl ExactSizeIterator<Item = EntryPoint>
+    pub fn entry_points(&self) -> impl ExactSizeIterator<Item = EntryPoint> + use<'_>
     {
         self.inner
             .entry_points()
@@ -218,8 +218,9 @@ impl<'a> ProgramReflection<'a>
         })
     }
 
-    pub fn entry_points(&self)
-    -> impl ExactSizeIterator<Item = EntryPointReflection<'a>>
+    pub fn entry_points(
+        &self,
+    ) -> impl ExactSizeIterator<Item = EntryPointReflection<'a>> + use<'a>
     {
         self.inner
             .entry_points()