Add .wrapping_offset() methods
.wrapping_offset() exposes the arith_offset intrinsic in the core
module (as methods on raw pointers, next to offset). 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".