Derivable Encode & Decode (#509)

* Derive macro for simple structs.

* Derive Encode/Decode wherever we can.

* Derive for enums.

* Support discriminant.

* Get rid of some repetition.

* Support custom indices.

* Derive codec for enums wherever possible.

* Fix no_std

* WASM rebuilt

* Avoid excessive import.

* Fix compilation.

* Address review grumbles.

polkadot/statement-table/src/lib.rs

@@ -18,6 +18,9 @@ extern crate substrate_codec as codec;
 extern crate substrate_primitives;
 extern crate polkadot_primitives as primitives;
 
+#[macro_use]
+extern crate substrate_codec_derive;
+
 pub mod generic;
 
 pub use generic::Table;