std: Stabilize a number of small APIs

The following APIs were all marked with a `#[stable]` tag:

* process::Child::id
* error::Error::is
* error::Error::downcast
* error::Error::downcast_ref
* error::Error::downcast_mut
* io::Error::get_ref
* io::Error::get_mut
* io::Error::into_inner
* hash::Hash::hash_slice
* hash::Hasher::write_{i,u}{8,16,32,64,size}

src/libstd/process.rs

@@ -505,7 +505,7 @@ impl Child {
     }
 
     /// Returns the OS-assigned process identifier associated with this child.
-    #[unstable(feature = "process_id", reason = "api recently added")]
+    #[stable(feature = "process_id", since = "1.3.0")]
     pub fn id(&self) -> u32 {
         self.handle.id()
     }