Use size_of_val instead of manual calculation

Very minor thing that I happened to notice in passing, but it's both shorter and means it gets `mul nuw`, so why not.

Author: scottmcm

core/src/hash/mod.rs

@@ -834,7 +834,7 @@ mod impls {
 
                 #[inline]
                 fn hash_slice<H: ~const Hasher>(data: &[$ty], state: &mut H) {
-                    let newlen = data.len() * mem::size_of::<$ty>();
+                    let newlen = mem::size_of_val(data);
                     let ptr = data.as_ptr() as *const u8;
                     // SAFETY: `ptr` is valid and aligned, as this macro is only used
                     // for numeric primitives which have no padding. The new slice only