nitg: fix FFI _is_null functions

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix generated signature for FFI cast

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: fix generated code for call to super from C

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge remote-tracking branch 'alexis/ffi-fix'

In most cases the generated code use a wrong types (Custom types in
extern classes aren't visible inside the main generated code). Also
fixed an alias.