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/example/src/lib.rs

@@ -121,9 +121,7 @@ decl_module! {
 /// circumstances that have happened that users, Dapps and/or chain explorers would find
 /// interesting and otherwise difficult to detect.
 decl_event!(
-	pub enum Event<T> with RawEvent<B>
-		where <T as balances::Trait>::Balance
-	{
+	pub enum Event<T> where B = <T as balances::Trait>::Balance {
 		// Just a normal `enum`, here's a dummy event to ensure it compiles.
 		/// Dummy event, just here so there's a generic type that's used.
 		Dummy(B),