Rollup merge of rust-lang#64836 - lzutao:stabilize-map_get_key_value, r=SimonSapin

Stabilize map_get_key_value feature

FCP done in rust-lang#49347 (comment)
r? @SimonSapin
Closes rust-lang#49347

Author: Centril

src/liballoc/collections/btree/map.rs

@@ -580,15 +580,14 @@ impl<K: Ord, V> BTreeMap<K, V> {
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_get_key_value)]
     /// use std::collections::BTreeMap;
     ///
     /// let mut map = BTreeMap::new();
     /// map.insert(1, "a");
     /// assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
     /// assert_eq!(map.get_key_value(&2), None);
     /// ```
-    #[unstable(feature = "map_get_key_value", issue = "49347")]
+    #[stable(feature = "map_get_key_value", since = "1.40.0")]
     pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>
         where K: Borrow<Q>,
               Q: Ord

src/libstd/collections/hash/map.rs

@@ -714,15 +714,14 @@ where
     /// # Examples
     ///
     /// ```
-    /// #![feature(map_get_key_value)]
     /// use std::collections::HashMap;
     ///
     /// let mut map = HashMap::new();
     /// map.insert(1, "a");
     /// assert_eq!(map.get_key_value(&1), Some((&1, &"a")));
     /// assert_eq!(map.get_key_value(&2), None);
     /// ```
-    #[unstable(feature = "map_get_key_value", issue = "49347")]
+    #[stable(feature = "map_get_key_value", since = "1.40.0")]
     #[inline]
     pub fn get_key_value<Q: ?Sized>(&self, k: &Q) -> Option<(&K, &V)>
     where