Runtime agnostic Events (#20)

* Introduce OpaqueEvent

* Look up event by module and variant

* Index events by module

* Get events by module

* Dynamically decode events

* Decode System events and EventRecord topics

* Use type sizes to decode raw events

* Remove unused imports

* rustfmt

* Unify error types, fix some compiler errors

* Make dynamic event decoding work

- fix compilation errors
- skip modules with no events when indexing
- preallocate vec for raw event data

* Remove printlns, replace where required with log

* Remove unused import

* Check missing type sizes

* Ignore unknown event arg type sizes

* Decode concrete System events, assumes every Runtime has the module

* Reorganise usings

* pub use some types

* Code docs

* Export Error

* Error Display impls

* Format code

src/srml/contracts.rs

@@ -115,3 +115,10 @@ where
             .call("call", (dest, compact(value), compact(gas_limit), data))
     }
 }
+
+/// Contracts Events
+#[derive(parity_scale_codec::Decode)]
+pub enum Event<T: System> {
+    /// Contract code stored
+    CodeStored(T::Hash),
+}