Revert "Remove usages of case(_) { fail; } since the compiler does this automatically". When we have exhaustiveness checking, "case(_) { fail; }" will be useful to silence warnings.

This reverts commit 92a716d862.

src/lib/deque.rs

@@ -47,6 +47,7 @@ fn create[T]() -> t[T] {
     fn get[T](vec[cell[T]] elts, uint i) -> T {
         alt (elts.(i)) {
             case (option.some[T](?t)) { ret t; }
+            case (_) { fail; }
         }
         fail;   // FIXME: remove me when exhaustiveness checking works
     }

src/lib/map.rs

@@ -176,6 +176,7 @@ fn mk_hashmap[K, V](&hashfn[K] hasher, &eqfn[K] eqer) -> hashmap[K, V] {
             fn get(&K key) -> V {
                 alt (find_common[K, V](hasher, eqer, bkts, nbkts, key)) {
                     case (option.some[V](?val)) { ret val; }
+                    case (_) { fail; }
                 }
                 fail;   // FIXME: remove me when exhaustiveness checking works
             }