Re-introduce zero copy codec and add minimal polkadot client API which uses linked native runtime (#65)

* client-api type and move duty roster types to primitives

* tuple implementation for slicable

* mild cleanup of deserialization code

* stubs which handle encoding and decoding themselves

* fancier impl_stubs macro

* zero-copy slicable API

* minimal polkadot-client API

* fix WASM API generation

* move native environment stuff to substrate executor

* fix warnings and grumbles

substrate/polkadot/cli/src/genesis.rs

@@ -89,7 +89,7 @@ mod tests {
 				"execute_transaction",
 				&vec![].join(&header).join(tx)
 			).unwrap();
-			header = Header::from_slice(&mut &ret_data[..]).unwrap();
+			header = Header::decode(&mut &ret_data[..]).unwrap();
 		}
 
 		let ret_data = execute(
@@ -99,7 +99,7 @@ mod tests {
 			"finalise_block",
 			&vec![].join(&header)
 		).unwrap();
-		header = Header::from_slice(&mut &ret_data[..]).unwrap();
+		header = Header::decode(&mut &ret_data[..]).unwrap();
 
 		(vec![].join(&Block { header, transactions }), hash.into())
 	}

substrate/polkadot/cli/src/lib.rs

@@ -83,7 +83,7 @@ pub fn run<I, T>(args: I) -> error::Result<()> where
 		storage = genesis_config.genesis_map();
 		let block = genesis::construct_genesis_block(&storage);
 		storage.extend(additional_storage_with_genesis(&block));
-		(primitives::block::Header::from_slice(&mut block.header.to_vec().as_ref()).expect("to_vec() always gives a valid serialisation; qed"), storage.into_iter().collect())
+		(primitives::block::Header::decode(&mut block.header.to_vec().as_ref()).expect("to_vec() always gives a valid serialisation; qed"), storage.into_iter().collect())
 	};
 	let client = client::new_in_mem(executor, prepare_genesis)?;