lib: adds missing import of math.h in standard library

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
contrib/sort_downloads: fix missing C include

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: changes the behavior of Coll::rand to behave like first

It will now return a non-nullable and `abort` when empty.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>