rational.hpp imports header iostream even if stream operators are not needed

[Levente Hunyadi <levente@…>]

Many Boost libraries have header variants with and without iostream support:

#include "boost/date_time/posix_time/posix_time.hpp" //include all types plus i/o

or

#include "boost/date_time/posix_time/posix_time_types.hpp" //no i/o just types

Likewise, there exists a macro BOOST_NO_IOSTREAM, which should prevent <iostream> from being included, see ticket #4060.

Neither approaches are available in boost::rational<> but either could be put to good use.

[Daryle Walker]

Looking at Config's documentation, it seems that BOOST_NO_IOSTREAM is descriptive, not proscriptive. It is meant to flag when an implementation doesn't have IOStreams installed (like free-standing environments), not disable IOStreams even when the compiler supports it.

Rational's implementation is just one header file, so separating I/O will make things more complex, as opposed to libraries that are already a bunch of headers and so one more won't make too much of a difference.

[Daryle Walker]

(In [85498]) Make the I/O part of Boost.Rational conditional on the configuration flag BOOST_NO_IOSTREAM (refs #7250).