std: Stabilize a number of small APIs

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}