nitc&lib: MapIterator keys can be nullable

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: update HashMap and ArrayMap to support nullable keys

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/map: add factory to Map returning a HashMap

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