[Integer] power_int_positive_int returns an option

This function relies on Z.power_int_positive_int, which can raise the
exception Invalid_argument when dealing with an exponent too big to
produce a valid result in GMP. This exception is now catched and thus
the function produce an option.

Modifications at callsites have been taken care of, and the file
floating_point.ml has been made more human readable.