* Implement CheckedBitPattern for core::num::NonZero*.
* Add tests for CheckedBitPattern.
* Fix stacked borrows violation.
`<&mut T>::as_ptr` gives a `*const T` which is read-only. `<&mut T>::as_mut_ptr` is necessary to get a `*mut T`.
* MSRV fix.
* Add layout checks for CheckedBitPattern for NonZero*.
(Until Rust guarantees the layout of NonZero[int] matches [int].)
Also adds a test that will fail if any NonZero[int] has a different size or alignment than [int].
