use pointer::add instead of pointer::offset (fix clippy warning)

for details see [the clippy warning](https://rust-lang.github.io/rust-clippy/master/index.html#/ptr_offset_with_cast).

Author: rursprung

src/murmur3.rs

@@ -137,7 +137,7 @@ impl core::hash::Hasher for Hasher {
                 let mid = 4 - index;
                 let head = unsafe { slice::from_raw_parts(bytes.as_ptr(), mid) };
                 let body = unsafe {
-                    slice::from_raw_parts(bytes.as_ptr().offset(mid as isize), len - mid)
+                    slice::from_raw_parts(bytes.as_ptr().add(mid), len - mid)
                 };
 
                 // NOTE(unsafe) avoid calling `memcpy` on a 0-3 byte copy