libstd: set baseline stability levels.

Earlier commits have established a baseline of `experimental` stability
for all crates under the facade (so their contents are considered
experimental within libstd). Since `experimental` is `allow` by
default, we should use the same baseline stability for libstd itself.

This commit adds `experimental` tags to all of the modules defined in
`std`, and `unstable` to `std` itself.

src/libstd/task.rs

@@ -91,6 +91,8 @@
 //! # }
 //! ```
 
+#![experimental]
+
 use any::Any;
 use comm::channel;
 use io::{Writer, stdio};