Implement __repr__ instead of __str__ for _NumericField

__repr__ provides a generic fallback for __str__ and, in the
case of IntegerFields the representation is both human readable
and unambiguous.

Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>