Add `BOOST_DEFINE_STRUCT`

[denzor200]

Like this:

BOOST_DEFINE_STRUCT(X, (), (
    (int, m1),
    (int, m2)
));

Gives the similar result as here: https://www.boost.org/doc/libs/master/libs/describe/doc/html/describe.html#classes_class_types_with_public_members