Make decl_event! more ergonomic (#723)

* Improve and simplify `decl_event!`

- RawEvent is not required anymore to be written in the defintion
- Events with and without parameters are now supported everywhere

* Make `decl_event!` more flexible with the automatic naming of generic parameters

The macro will now automatically derive the name of a generic parameter
from the trait type name, if no explicit name is given.

`where Balance = <T as Trait>::Balance` can be simplified to
`where <T as Trait>::Balance`.

* Adapts to latest refactoring changes

substrate/srml/support/src/metadata.rs

@@ -151,8 +151,7 @@ mod tests {
 		}
 
 		decl_event!(
-			pub enum Event<T> with RawEvent<Balance>
-				where <T as Trait>::Balance
+			pub enum Event<T> where <T as Trait>::Balance
 			{
 				/// Hi, I am a comment.
 				TestEvent(Balance),
@@ -179,8 +178,7 @@ mod tests {
 		}
 
 		decl_event!(
-			pub enum Event<T> with RawEvent<Balance>
-				where <T as Trait>::Balance
+			pub enum Event<T> where <T as Trait>::Balance
 			{
 				TestEvent(Balance),
 			}