std: Move overflowing ops to inherent methods

This commit migrates all of the methods on `num::wrapping::OverflowingOps` onto
inherent methods of the integer types. This also fills out some missing gaps in
the saturating and checked departments such as:

* `saturating_mul`
* `checked_{neg,rem,shl,shr}`

This is done in preparation for stabilization,

cc rust-lang#27755

Author: alexcrichton