Improve the typing for restricted block constructs

[Drup]

This strengthen a bit the typing for dt, address and header elements that
are not supposed to be nested.

In particular, this was allowed by tyxml (where it shouldn't):

let x = body [dl [dd [txt "foo"]; dt [a [h1 [txt "foo"]]]]]

More importantly (and that's the main reason for actually fixing this),
the coercion flow5_without_sectioning_heading_header_footer :> flow5_without_header_footer was failing which was extremely annoying in practice.

[Drup]

I just realized this is not really backward compatible, which is mildly annoying to say the least. I'm wondering if anyone is affected except odoc.