Add operators

[pdimov]

Add a namespace boost::describe::operators containing ==, !=, <, <=, >, >=. Typical use would be

namespace user
{

struct X
{
};

BOOST_DESCRIBE_STRUCT(X, (), ())

using boost::describe::operators::operator==;
using boost::describe::operators::operator!=;

} // namespace user

Maybe add boost::describe::eq, boost::describe::lt, boost::describe::le as helper functions.

Could the operators be directly in boost::describe? (Probably not.)

[pdimov]

Added.

[ddevienne]

Just curious Peter, but why aren't the commits associated to GH Issues?

[pdimov]

No particular reason; there's no guarantee that things will be implemented in a single commit, although I could have used "refs".