Make constants exposable in metadata (#2975)

* Some cleanup

* Add module constant metadata declaration

* Begin to integrate the constants in `decl_module`

* Fixes tests

* Fix compilation and add tests

* Remove duplicate code

* Expose constants in democracy and staking + further fixes

* Update srml/metadata/src/lib.rs

Co-Authored-By: YJ <yjkimjunior@gmail.com>

* Hide `RawEvent` metadata function

* Prevent whitespaces in types

* Fix `offchain_worker` and `constants` with instances

* Up the `impl_version`

* Fix macro

* Incrase impl_version

substrate/srml/support/src/event.rs

@@ -136,6 +136,7 @@ macro_rules! decl_event {
 		}
 		impl Event {
 			#[allow(dead_code)]
+			#[doc(hidden)]
 			pub fn metadata() -> &'static [ $crate::event::EventMetadata ] {
 				$crate::__events_to_metadata!(; $( $events )* )
 			}