This avoids some runtime tests. The rest of the module makes
similar assumptions and there is little point to testing here.
* lib/timespec.h: Include verify.h instead of assure.h.
(timespec_cmp): Use ‘assume’, not ‘assure’.
Also, remove an unnecessary cast to ‘int’, as lots of other
code in this module now causes -Wconversion to complain, and
this is a problem with -Wconversion not with the code.
* modules/timespec (Depends-on): Depend on ‘verify’, not ‘assure’.