miscellaneous pure annotations and other small changes.

it seems that, to be truly useful, pure fns really need the
ability to modify their parameters.  alternatively, we could
rewrite the functions that modify their arguments to take/return.

Author: nikomatsakis

src/libcore/rand.rs

@@ -218,7 +218,7 @@ impl extensions for rng {
     }
 
     #[doc = "Shuffle a mutable vec in place"]
-    fn shuffle_mut<T>(&values: [mut T]) {
+    fn shuffle_mut<T>(&&values: [mut T]) {
         let mut i = values.len();
         while i >= 2u {
             // invariant: elements with index >= i have been locked in place.