std: Move rand to librand.

This functionality is not super-core and so doesn't need to be included
in std. It's possible that std may need rand (it does a little bit now,
for io::test) in which case the functionality required could be moved to
a secret hidden module and reexposed by librand.

Unfortunately, using #[deprecated] here is hard: there's too much to
mock to make it feasible, since we have to ensure that programs still
typecheck to reach the linting phase.

src/test/compile-fail/task-rng-isnt-sendable.rs

@@ -10,9 +10,11 @@
 
 // ensure that the TaskRng isn't/doesn't become accidentally sendable.
 
+extern crate rand;
+
 fn test_send<S: Send>() {}
 
 pub fn main() {
-    test_send::<::std::rand::TaskRng>();
-    //~^ ERROR: incompatible type `std::rand::TaskRng`, which does not fulfill `Send`
+    test_send::<::rand::TaskRng>();
+    //~^ ERROR: incompatible type `rand::TaskRng`, which does not fulfill `Send`
 }