Auto merge of #30523 - ubsan:wrapping_op_assign, r=eddyb

Add OpAssign to Wrapping<T>, plus fix some problems in core::num::wrapping

including, but not limited to:

* Testing Wrapping<T>
* Pull out a lot of broken code that doesn't need to be there with the new stage0 compiler
* Adding Rem and RemAssign to Wrapping<T>
* Removed 3 (assumed accidental) re-exports, which is a minor [breaking-change].
* Change shl and shr to take all integer types, instead of a usize; this is a more major [breaking-change], because of values that were inferred before, but brings us in line with the integer shifts.

Fixes #30524 and #30523

Author: bors