Support for constructors

[pdimov]

BOOST_DESCRIBE_CONSTRUCTORS(X, (int), (float, float), (X const&), (X&&)), mod_constructor.

At the moment the workaround is https://godbolt.org/z/TxYKs5Wfr, but it would be better to support this natively.

[RichardLuo0]

I would like to have this feature. This can help me solve this: https://godbolt.org/z/GG1oK3vbj.
Why is here no activity since 2022?

[pdimov]

Why is here no activity since 2022?

I've been busy with other things, sorry.

I'll try to get back to this before 1.86 ships.