Add a len function to GenerateToAddress

Add a convenience function to the `GenerateToAddress` model type to get
the number of blocks generated.

Author: tcharding

json/src/model/generating.rs

@@ -11,3 +11,8 @@ use serde::{Deserialize, Serialize};
 /// Models the result of JSON-RPC method `generatetoaddress`.
 #[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
 pub struct GenerateToAddress(pub Vec<BlockHash>);
+
+impl GenerateToAddress {
+    /// Returns the number of blocks generated.
+    pub fn len(&self) -> usize { self.0.len() }
+}