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/test/run-pass/drop-with-type-ascription-1.rs

@@ -9,11 +9,9 @@
 // except according to those terms.
 
 
-#![feature(str_words)]
-
 fn main() {
     let foo = "hello".to_string();
-    let foo: Vec<&str> = foo.words().collect();
+    let foo: Vec<&str> = foo.split_whitespace().collect();
     let invalid_string = &foo[0];
     assert_eq!(*invalid_string, "hello");
 }