ni: adds system to manage native local references to Nit objects

ni: adds system to manage native local references to Nit objects

This ensures that references to Nit objects from C within an extern
method are preserved when the GC is invoqued. Theses references
are registered automatically when any callback to the Nit system is
made. They are registered in a stack, local to the currently executing
extern method (ie the one on the top of the execution stack). All
registered references are cleared on the function return.

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