make offset must_use

https://djugei.github.io/bad-at-unsafe/ describes an error a user had when trying to use offset:

> At first I just assumed that the .add() and .offset() methods on pointers would mutate the pointer. They do not. Instead they return a new pointer, which gets dropped silently if you don't use it. Unlike for example Result, which is must_use annotated.

Author: steveklabnik

src/libcore/intrinsics.rs

@@ -1311,6 +1311,7 @@ extern "rust-intrinsic" {
     ///
     /// The stabilized version of this intrinsic is
     /// [`std::pointer::offset`](../../std/primitive.pointer.html#method.offset).
+    #[must_use = "returns a new pointer rather than modifying its argument"]
     pub fn offset<T>(dst: *const T, offset: isize) -> *const T;
 
     /// Calculates the offset from a pointer, potentially wrapping.