Specialize HashStable for [u8] slices

Particularly for ctfe-stress-4, the hashing of byte slices as part of the
MIR Allocation is quite hot. Previously, we were falling back on byte-by-byte
copying of the slice into the SipHash buffer (64 bytes long) before hashing a 64
byte chunk, and then doing that again and again.

This should hopefully be an improvement for that code.

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -301,6 +301,13 @@ impl<T: HashStable<CTX>, CTX> HashStable<CTX> for [T] {
     }
 }
 
+impl<CTX> HashStable<CTX> for [u8] {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
+        self.len().hash_stable(ctx, hasher);
+        hasher.write(self);
+    }
+}
+
 impl<T: HashStable<CTX>, CTX> HashStable<CTX> for Vec<T> {
     #[inline]
     fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {