Rename TaskRng to ThreadRng

Since runtime is removed, rust has no tasks anymore and everything is moving
from being task-* to thread-*. Let’s rename TaskRng as well!

* Rename TaskRng to ThreadRng
* Rename task_rng to thread_rng

[breaking-change]

src/libstd/os.rs

@@ -1438,7 +1438,7 @@ mod tests {
     }
 
     fn make_rand_name() -> String {
-        let mut rng = rand::task_rng();
+        let mut rng = rand::thread_rng();
         let n = format!("TEST{}", rng.gen_ascii_chars().take(10u)
                                      .collect::<String>());
         assert!(getenv(n.as_slice()).is_none());