tests: adds an extra simple test for the FFI

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: adds an FFI test with simple callbacks

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: adds an FFI test with different callbacks

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: adds an FFI test with strings

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
examples: converts examples/extern_methods to use FFI

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: converts curses.nit to use the FFI

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