bevy_reflect: streamline generated FromReflect::from_reflect (#19906)

# Objective

Generated `from_reflect` methods use closures in a weird way, e.g.:
```rust
    x: (|| {
        <f32 as ::bevy::reflect::FromReflect>::from_reflect(
            ::bevy::reflect::Struct::field(__ref_struct, "x")?,
        )   
    })()?,
```
The reason for this is because when `#[reflect(Default)]` is used, you
instead get stuff like this:
```rust
    if let ::core::option::Option::Some(__field) = (|| {
        <f32 as ::bevy::reflect::FromReflect>::from_reflect(
            ::bevy::reflect::Struct::field(__ref_struct, "x")?,
        )   
    })() {
        __this.x = __field;
    } 
```
and the closure is necessary to contain the scope of the `?`. But the
first case is more common.

Helps with #19873.

## Solution

Avoid the closure in the common case.

## Testing

I used cargo expand to confirm the closures are no longer produced in
the common case.

`-Zmacro-stats` output tells me this reduces the size of the `Reflect`
code produced for `bevy_ui` by 0.5%.

Author: nnethercote

crates/bevy_reflect/derive/src/from_reflect.rs

@@ -146,7 +146,8 @@ fn impl_struct_internal(
         quote! {
             let mut #__this = <#reflect_ty as #FQDefault>::default();
             #(
-                if let #fqoption::Some(__field) = #active_values() {
+                // The closure catches any failing `?` within `active_values`.
+                if let #fqoption::Some(__field) = (|| #active_values)() {
                     // Iff field exists -> use its value
                     #__this.#active_members = __field;
                 }
@@ -158,7 +159,7 @@ fn impl_struct_internal(
 
         quote! {
             let #__this = #constructor {
-                #(#active_members: #active_values()?,)*
+                #(#active_members: #active_values?,)*
                 #(#ignored_members: #ignored_values,)*
             };
             #FQOption::Some(#retval)
@@ -274,35 +275,31 @@ fn get_active_fields(
                             <#ty as #bevy_reflect_path::FromReflect>::from_reflect(field)
                         });
                         quote! {
-                            (||
-                                if let #FQOption::Some(field) = #get_field {
-                                    #value
-                                } else {
-                                    #FQOption::Some(#path())
-                                }
-                            )
+                            if let #FQOption::Some(field) = #get_field {
+                                #value
+                            } else {
+                                #FQOption::Some(#path())
+                            }
                         }
                     }
                     DefaultBehavior::Default => {
                         let value = into_remote(quote! {
                             <#ty as #bevy_reflect_path::FromReflect>::from_reflect(field)
                         });
                         quote! {
-                            (||
-                                if let #FQOption::Some(field) = #get_field {
-                                    #value
-                                } else {
-                                    #FQOption::Some(#FQDefault::default())
-                                }
-                            )
+                            if let #FQOption::Some(field) = #get_field {
+                                #value
+                            } else {
+                                #FQOption::Some(#FQDefault::default())
+                            }
                         }
                     }
                     DefaultBehavior::Required => {
                         let value = into_remote(quote! {
                             <#ty as #bevy_reflect_path::FromReflect>::from_reflect(#get_field?)
                         });
                         quote! {
-                            (|| #value)
+                            #value
                         }
                     }
                 };