Deprecate unfold for std::iter::from_fn

Author: Philippe-Cholet

src/sources.rs

@@ -98,6 +98,7 @@ where
 ///                         vec![1, 1, 2, 3, 5, 8, 13, 21]);
 /// assert_eq!(fibonacci.last(), Some(2_971_215_073))
 /// ```
+#[deprecated(note = "Use std from_fn() instead", since = "0.13.0")]
 pub fn unfold<A, St, F>(initial_state: St, f: F) -> Unfold<St, F>
 where
     F: FnMut(&mut St) -> Option<A>,
@@ -118,6 +119,7 @@ where
 /// See [`unfold`](crate::unfold) for more information.
 #[derive(Clone)]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
+#[deprecated(note = "Use std from_fn() instead", since = "0.13.0")]
 pub struct Unfold<St, F> {
     f: F,
     /// Internal state that will be passed to the closure on the next iteration