add method add_data as a shortcut to create an OP_RETURN output, fix the dust check to consider only spendable output

src/wallet/tx_builder.rs

@@ -560,6 +560,13 @@ impl<'a, B, D: BatchDatabase, Cs: CoinSelectionAlgorithm<D>> TxBuilder<'a, B, D,
         self
     }
 
+    /// Add data as an output, using OP_RETURN
+    pub fn add_data(&mut self, data: &[u8]) -> &mut Self {
+        let script = Script::new_op_return(data);
+        self.add_recipient(script, 0u64);
+        self
+    }
+
     /// Sets the address to *drain* excess coins to.
     ///
     /// Usually, when there are excess coins they are sent to a change address generated by the