implement rfc 1054: split_whitespace() fn, deprecate words()

For now, words() is left in (but deprecated), and Words is a type alias for
struct SplitWhitespace.

Also cleaned up references to s.words() throughout codebase.

Closes #15628

src/librustdoc/html/markdown.rs

@@ -274,7 +274,7 @@ pub fn render(w: &mut fmt::Formatter, s: &str, print_toc: bool) -> fmt::Result {
         };
 
         // Transform the contents of the header into a hyphenated string
-        let id = s.words().map(|s| s.to_ascii_lowercase())
+        let id = s.split_whitespace().map(|s| s.to_ascii_lowercase())
             .collect::<Vec<String>>().connect("-");
 
         // This is a terrible hack working around how hoedown gives us rendered