Tweak some stabilizations in libstd

This commit tweaks a few stable APIs in the `beta` branch before they hit
stable. The `str::is_whitespace` and `str::is_alphanumeric` functions were
deleted (added in #49381, issue at #49657). The `and_modify` APIs added
in #44734 were altered to take a `FnOnce` closure rather than a `FnMut` closure.

Closes #49581
Closes #49657

src/libstd/collections/hash/map.rs

@@ -2127,8 +2127,8 @@ impl<'a, K, V> Entry<'a, K, V> {
     /// assert_eq!(map["poneyland"], 43);
     /// ```
     #[stable(feature = "entry_and_modify", since = "1.26.0")]
-    pub fn and_modify<F>(self, mut f: F) -> Self
-        where F: FnMut(&mut V)
+    pub fn and_modify<F>(self, f: F) -> Self
+        where F: FnOnce(&mut V)
     {
         match self {
             Occupied(mut entry) => {