Sleep approximately self seconds

Is not interrupted by signals.

Property definitions

core :: time $ Float :: sleep
	# Sleep approximately `self` seconds
	#
	# Is not interrupted by signals.
	fun sleep `{
		time_t s = self;
		long ns = (self-s) * 1000000000.0;
		struct timespec req = {s, ns};

		while (nanosleep(&req, &req)) { }
	`}
lib/core/time.nit:62,2--71,3