contracts: Run start function (#1367)

Fixes #116 

Start function wasn't allowed in a contract. Now it is allowed and is
being run.

It was disallowed because it is not used by Rust and supporting it made
the code more complex. However, not running the start function violates
the wasm standard. This makes life harder for some languages (see linked
ticket).

substrate/frame/contracts/src/benchmarking/sandbox.rs

@@ -58,7 +58,13 @@ impl<T: Config> From<&WasmModule<T>> for Sandbox {
 			.add_fuel(u64::MAX)
 			.expect("We've set up engine to fuel consuming mode; qed");
 
-		let entry_point = instance.get_export(&store, "call").unwrap().into_func().unwrap();
+		let entry_point = instance
+			.start(&mut store)
+			.unwrap()
+			.get_export(&store, "call")
+			.unwrap()
+			.into_func()
+			.unwrap();
 		Self { entry_point, store }
 	}
 }