Add constexpr to some functions in Boost.Rational
Added cross-instantiation constructor template, refs #9018.
Make the I/O part of Boost.Rational conditional on the configuration flag BOOST_NO_IOSTREAM (refs #7250).
Started modernizing Boost.Rational I/O (refs #7187)
Fixed normalization problem with Boost.Rational's "assign"; add check for negative values too large for normalization (refs #9067)
Integrated Boost.Rational's sanity checks during assignment in the input operator, respecting IOStreams' exception policy