[m-rev.] for review: add a builtin unsigned word sized integer type -- Part 1

[Sebastian Godelet]

Hi Julien

> I won't commit this until I've finished part 2 of this change and posted
> that for review.  Once it is committed everyone will need to update
> their installed compilers before part 2 is commited.
> 
Can I use a recent compiler to compile this commit or would I have to re-bootstrap?

This change does not cover anything along the lines of exposing int32_t and the likes to the C backend, right?
Now that we can require C99, that means we can also make use of <inttypes.h>, or is that an not always the case?

Cheers,

Sebastian.