core: Clean up crate docs and give all mods a brief description

src/libcore/util.rs

@@ -1,13 +1,15 @@
+/*!
+
+Miscellaneous helpers for common patterns.
+
+*/
+
 // NB: transitionary, de-mode-ing.
 #[forbid(deprecated_mode)];
 #[forbid(deprecated_pattern)];
 
 use cmp::Eq;
 
-/**
- * Miscellaneous helpers for common patterns.
- */
-
 /// The identity function.
 #[inline(always)]
 pure fn id<T>(+x: T) -> T { move x }