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/error.rs

@@ -14,22 +14,28 @@
 // You should have received a copy of the GNU General Public License
 // along with substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
 
-use crate::metadata::MetadataError;
+use crate::{
+    events::EventsError,
+    metadata::MetadataError,
+};
 use jsonrpc_core_client::RpcError;
 use parity_scale_codec::Error as CodecError;
 use std::io::Error as IoError;
 use substrate_primitives::crypto::SecretStringError;
 
 /// Error enum.
-#[derive(Debug, derive_more::From)]
+#[derive(Debug, derive_more::From, derive_more::Display)]
 pub enum Error {
     /// Codec error.
     Codec(CodecError),
+    /// Events error.
+    Events(EventsError),
     /// Io error.
     Io(IoError),
     /// Rpc error.
     Rpc(RpcError),
     /// Secret string error.
+    #[display(fmt = "Secret String Error")]
     SecretString(SecretStringError),
     /// Metadata error.
     Metadata(MetadataError),