Make `ObjectClass` and `Handle` public

[Berrysoft]

I'm the author of crate hdf5-hl. I would like to use this maintained fork, and I would like to request for this new feature: make trait ObjectClass and type Handle public. Then I can remove transmutes from my crate.

[Berrysoft]

OK, I use class_id to solve the transmute. But class_id doesn't increase the ref count. I still would like such a function which also increases the ref count.

[magnusuMET]

This requires some heavy thinking on safety of creating the handles, I won't have enough time to look at in for quite some time

[Berrysoft]

That's OK. I'm not worry about that.