nitg & niti: add intern methods `successor` and `predecessor`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
c_src: update with new intern methods

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: intro the Numeric class

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
nitg & niti: remove intern methods succ, prec and Char::+|-

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: move up to_f and to_i in Number

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib: intro Number::zero and Number::is_zero

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
tests: migrate tests using Char::+|- and Int::succ|prec

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