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/librustdoc/test.rs

@@ -437,7 +437,7 @@ fn partition_source(s: &str) -> (String, String) {
 
     for line in s.lines() {
         let trimline = line.trim();
-        let header = trimline.is_whitespace() ||
+        let header = trimline.chars().all(|c| c.is_whitespace()) ||
             trimline.starts_with("#![") ||
             trimline.starts_with("#[macro_use] extern crate") ||
             trimline.starts_with("extern crate");