APER: fix non negative normally small whole numbers

[CedricRouxEurecom]

aper_put_nsnnwn() and aper_get_nsnnwn() were wrong. All the places where they were used are modified, if needed.

The functions aper_get_constrained_whole_number() and aper_put_constrained_whole_number() are introduced. They don't cover all cases, very big numbers are not supported.