core: Revive SipHash's tests

These tests were bitrotting, include them in the crate and bring them up
to date and compiling.. and they pass.

src/libcore/hash/sip.rs

@@ -10,8 +10,6 @@
 
 //! An implementation of SipHash 2-4.
 
-#![allow(deprecated)] // until the next snapshot for inherent wrapping ops
-
 use prelude::*;
 use super::Hasher;