Add Script:dust_value() to get minimum output value for a spk

src/blockdata/script.rs

@@ -325,6 +325,16 @@ impl Script {
                                opcodes::All::from(self.0[0]).classify() == opcodes::Class::IllegalOp)
     }
 
+    /// Gets the minimum value an output with this script should have in order to be
+    /// broadcastable on today's bitcoin network.
+    pub fn dust_value() {
+        if self.is_witness_program() {
+            294
+        } else {
+            546
+        }
+    }
+
     /// Iterate over the script in the form of `Instruction`s, which are an enum covering
     /// opcodes, datapushes and errors. At most one error will be returned and then the
     /// iterator will end. To instead iterate over the script as sequence of bytes, treat