Make BitWidth use NonZeroUsize

[AaronKutch]

The compiler has special optimizations for this. In fact, I might just replace BitWidth altogether, but I will have to try that in an experimental branch in the future.

[Robbepop]

I was also already thinking about this and also to use other things such as MaybeUninit, ManuallyDrop. I think apint could make use a lot more of those new abstractions.

[AaronKutch]

Yeah. I learned a bunch about manipulating pointers and using MaybeUninit when working on my ptr_rotate PR, so I will do this.

[AaronKutch]

I was considering replacing BitWidth altogether with NonZeroUsize, but I have decided to keep it as a wrapper type around a NonZeroUsize. I found it unlikely that we would use unused bits in the len, but someone might want to clone the library and use NonZeroU32 or something else special.