Attempt at removing the kludge concerning non-standard printf format references

 e61eae4 tries to make the `printf` format references to `size_t` arguments more portable with `#ifdef`s around every occurrence. I find this code duplication verbose and kludgey. In the spirit of inttypes.h I introduce a set of macros `PRI[ux]SIZE` for these format references.

I'm unsure whether the new header file `neo/portability/inttypes.h` is placed and referred to reasonably and would be glad about better suggestions.