lib: fix import of stdio.h for String::to_precision

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: add missing copyright info to test_ffi_c_simple

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: fix missing code in test_ffi_c_callback_extern_receiver

Some C includes and Nit callbacks were missing.

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: converts 6 tests from the old nitni to FFI

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