+ sub-second sleep()? alarm()? time()? (integrate Time::HiRes?
+ Configure doesn't yet probe for usleep/nanosleep/ualarm but
+ the units exist)
+ floating point handling: nans, infinities, fp exception masks, etc.
+ At least the following interfaces exist: fp_classify(), fp_class(),
+ class(), isinf(), isfinite(), finite(), isnormal(), unordered(),
+ <ieeefp.h>, <fp_class.h> (there are metaconfig units for all these),
+ fp_setmask(), fp_getmask(), fp_setround(), fp_getround()
+ (no metaconfig units yet for these).
+ Don't forget finitel(), fp_classl(), fp_class_l(), (yes, both do,
+ unfortunately, exist), and unorderedl().
+ PREFERABLY AS AN EXTENSION.
+ As of 5.6.1 there is cpp macro Perl_isnan().
+ fix the basic arithmetics (+ - * / %) to preserve IVness/UVness if
+ both arguments are IVs/UVs: it sucks that one cannot see
+ the 'carry flag' (or equivalent) of the CPU from C,
+ C is too high-level...