[Rust] Add Workflow::cloned associated function

This simplifies the very common pattern of retrieving a workflow then cloning it in order to register activities.

Author: bdash

rust/src/workflow.rs

@@ -280,6 +280,10 @@ impl Workflow {
         unsafe { Workflow::ref_from_raw(NonNull::new(result).unwrap()) }
     }
 
+    pub fn cloned(name: &str) -> Ref<Workflow> {
+        Self::instance(name).clone_to(name)
+    }
+
     /// List of all registered [Workflow]'s
     pub fn list() -> Array<Workflow> {
         let mut count = 0;