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/tests.rs

@@ -862,6 +862,27 @@ fn deposit_event_max_value_limit() {
 	});
 }
 
+// Fail out of fuel (ref_time weight) inside the start function.
+#[test]
+fn run_out_of_fuel_start_fun() {
+	let (wasm, _code_hash) = compile_module::<Test>("run_out_of_gas_start_fn").unwrap();
+	ExtBuilder::default().existential_deposit(50).build().execute_with(|| {
+		let _ = <Test as Config>::Currency::set_balance(&ALICE, 1_000_000);
+		assert_err_ignore_postinfo!(
+			Contracts::instantiate_with_code(
+				RuntimeOrigin::signed(ALICE),
+				0,
+				Weight::from_parts(1_000_000_000_000, u64::MAX),
+				None,
+				wasm,
+				vec![],
+				vec![],
+			),
+			Error::<Test>::OutOfGas,
+		);
+	});
+}
+
 // Fail out of fuel (ref_time weight) in the engine.
 #[test]
 fn run_out_of_fuel_engine() {