nitg: intro the nitni module

Offers services defining the tailored API used by the FFI.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: intro the common_ffi module and support for the C language

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: intro the compiler_ffi module to support the FFI in nitg

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg-g: support cast between subtypes of Pointer

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg: activate support for the FFI in nitg

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

Conflicts:
	src/abstract_compiler.nit
	src/separate_compiler.nit
nitg: detect if a compiler supports the FFI

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge branch 'ffi_core' into merge_ffi