Make Transaction Field in V4 Slate Optional (#356)

* make transaction field in V4 Slate optional

* add mutable/non mutable version of tx getter

* removal of mut references where not needed

* remove more muts

* update from master

* update from master

* test fixes

controller/tests/transaction.rs

@@ -120,9 +120,14 @@ fn basic_transaction_api(test_dir: &'static str) -> Result<(), libwallet::Error>
 		slate = sender_api.finalize_tx(m, &slate)?;
 
 		// Check we have a single kernel and that it is a Plain kernel (no lock_height).
-		assert_eq!(slate.tx.kernels().len(), 1);
+		assert_eq!(slate.tx_or_err()?.kernels().len(), 1);
 		assert_eq!(
-			slate.tx.kernels().first().map(|k| k.features).unwrap(),
+			slate
+				.tx_or_err()?
+				.kernels()
+				.first()
+				.map(|k| k.features)
+				.unwrap(),
 			transaction::KernelFeatures::Plain { fee: 2000000 }
 		);
 
@@ -170,7 +175,7 @@ fn basic_transaction_api(test_dir: &'static str) -> Result<(), libwallet::Error>
 
 	// post transaction
 	wallet::controller::owner_single_use(Some(wallet1.clone()), mask1, None, |api, m| {
-		api.post_tx(m, &slate.tx, false)?;
+		api.post_tx(m, slate.tx_or_err()?, false)?;
 		Ok(())
 	})?;