mark std::str::replacen and std::str::replace as #[must_use].

src/liballoc/lib.rs

@@ -96,6 +96,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(exact_size_is_empty)]
 #![feature(fmt_internals)]
+#![feature(fn_must_use)]
 #![feature(from_ref)]
 #![feature(fundamental)]
 #![feature(lang_items)]