Adding rotate_through_iterators for select_phantom_hints refactor

- adding function to allow for select_phantom_hints to yield an iterator

Author: upjohnc

lightning-invoice/src/utils.rs

@@ -318,6 +318,8 @@ fn rotate_through_iterators<T, I: Iterator<Item = T>>(mut vecs: Vec<I>) -> impl
 			}
 		}
 	})
+}
+
 #[cfg(feature = "std")]
 /// Utility to construct an invoice. Generally, unless you want to do something like a custom
 /// cltv_expiry, this is what you should be using to create an invoice. The reason being, this