Module preamble/header detection.

[dbuenzli]

Related to #235

The way module preambles are detected is odd. Basically one expects to have anything that happens before the first header or structure item to go in the module preamble/header.

More precisely in the stdlib Int64 module I would expect all of the contents of the first comment to go in the preamble. Instead, it seems all of it put in an aside element (!?):

https://github.com/ocaml/ocaml/blob/4.07/stdlib/int64.mli#L16-L28
https://b0-system.github.io/odig/doc/ocaml/Stdlib/Int64/index.html

[dbuenzli]

Also I suspect this should explain the annoying lack of module synopses in the 4.08 docset here:

https://b0-system.github.io/odig/dev/ocaml/Stdlib/index.html#modules

[dbuenzli]

This is still an issue with the new odoc. Basically the module preamble here:

https://github.com/ocaml/ocaml/blob/4.09/stdlib/int64.mli#L16-L35

Ends up in div.odoc-content rather than header.odoc-preamble (using classification terminology from #589)

[dbuenzli]

I took the liberty to add this to the 2.0.0 milestone because it basically breaks all Stdlib module preambles and as a side effect their synopsis line in the Stdlib module.

[Julow]

I've made a reproduction test case for this issue: 113d006
This seems to be due to the stdlib__ wrapping of the stdlib. Generating the html for stdlib.odocl spits out a page for Int64 that has the module's documentation in the wrong place. The html for stdlib__int64.odocl has the documentation in the preamble (but the rest of the page is empty due to it being a "hidden" module).