decl_module! macro: use 'frame_system' instead of system as default ident (#6500)

* Use frame_system as default ident.

* Remove unused 'frame_system' to 'system' renaming.

* Fix construct_runtime_ui tests.

* Rename system to frame_system in sudo/utility pallet test.

* Bump runtime impl_version.

* Update formatting.

substrate/frame/support/test/tests/instance.rs

@@ -51,6 +51,7 @@ mod module1 {
 	frame_support::decl_module! {
 		pub struct Module<T: Trait<I>, I: InstantiableThing> for enum Call where
 			origin: <T as system::Trait>::Origin,
+			system = system,
 			T::BlockNumber: From<u32>
 		{
 			fn offchain_worker() {}
@@ -129,7 +130,8 @@ mod module2 {
 
 	frame_support::decl_module! {
 		pub struct Module<T: Trait<I>, I: Instance=DefaultInstance> for enum Call where
-			origin: <T as system::Trait>::Origin
+			origin: <T as system::Trait>::Origin,
+			system = system
 		{
 			fn deposit_event() = default;
 		}