[m-rev.] for post-commit review: add casts to and from uint32 and uint16

[Zoltan Somogyi]

On Mon, 3 Jan 2022 18:18:57 +1100 (AEDT), Julien Fischer <jfischer at opturion.com> wrote:
> For post-commit review by anyone.

The diff is fine as far as it goes, by I don't think
we should add such operations piecemeal, because this
can introduce gratuitous differences between {int,uint}*.m.
For example, after this diff, uint32.m contains casts to and from
uint8s and uint16s, but int32.m contains casts to and from
neither int8s or int16s.

I think if an operation exists in any of the uintN modules,
its corresponding version should also exist in the corresponding
intN module, provided that this corresponding version makes sense.
However, that is for future diffs.

Zoltan.