FFI: use libgc allocs

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
FFI: implement the global reference functions

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: test incr_ref with lots of short-lived (in Nit) objects

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