rustc: Remove matching on ~str from the language

The `~str` type is not long for this world as it will be superseded by the
soon-to-come DST changes for the language. The new type will be
`~Str`, and matching over the allocation will no longer be supported.
Matching on `&str` will continue to work, in both a pre and post DST world.

src/libstd/task.rs

@@ -387,8 +387,8 @@ fn test_back_to_the_future_result() {
 fn test_try_success() {
     match try(proc() {
         ~"Success!"
-    }) {
-        result::Ok(~"Success!") => (),
+    }).as_ref().map(|s| s.as_slice()) {
+        result::Ok("Success!") => (),
         _ => fail!()
     }
 }