(re-) Add pow functionality for rational

See: https://github.com/boostorg/rational/pull/24

In 2015 a pull request added pow() and reciprocal(). In 2018 it was reviewed by CMT and reciprocal() was found to be unnecessary. This left boost::pow() for rational, however it introduced an issue resolving std::pow from within boost namespaces. This happened close to the 1.68.0 release (during beta) so it was removed.

Originally recommend looking at adding `operator ^=` as a modifying operator to raise the rational value to an integer exponent. In addition `operator ^` could take an existing rational and raise it to an exponent, returning a new rational. - note comments below.