core::ptr: Add .wrapping_offset() methods

core::ptr: Add .wrapping_offset() methods

.wrapping_offset() exposes the arith_offset intrinsic in the core
module. This is the first step in making it possible to stabilize the
interface later.

`arith_offset` is a useful tool for developing iterators for two
reasons:

1. `arith_offset` is used by the slice's iterator, the most important
   iterator in libcore, and it is natural that Rust users need the same
   power available to implement similar iterators.
2. It is a good way to implement raw pointer iterations with step
   greater than one.

The name seems to fit the style of methods like "wrapping_add".