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/lib.rs

@@ -95,6 +95,7 @@
 //! and `format!`, also available to all Rust code.
 
 #![crate_id = "std#0.11.0-pre"]
+#![unstable]
 #![comment = "The Rust standard library"]
 #![license = "MIT/ASL2"]
 #![crate_type = "rlib"]