gpui: Reduce manual shifting & other minor improvements (#34407)

Minor cleanup in gpui.

- Reduce manual shifting by using `u32::to_be_bytes`
- Remove eager `Vec` allocation when listing registered actions
- Remove unnecessary return statements
- Replace manual `if let Some(_)` with `.as_deref_mut()`

Release Notes:

- N/A

crates/docs_preprocessor/src/main.rs

@@ -243,7 +243,6 @@ struct ActionDef {
 
 fn dump_all_gpui_actions() -> Vec<ActionDef> {
     let mut actions = gpui::generate_list_of_all_registered_actions()
-        .into_iter()
         .map(|action| ActionDef {
             name: action.name,
             human_name: command_palette::humanize_action_name(action.name),