Change int to uint in count_is constructor in extfmt

[catamorphism]

As per a FIXME in function pad. Again, I tried, but the magic of macros is beyond me. count_is always seems to be applied to non-negative ints, so changing to uint gets rid of some casts.

[catamorphism]

I have a fix for this now, just need to make a snapshot.

[catamorphism]

Fixed in a006608