latex backends: add an option to shorten submodules

[Octachron]

In the latex backend, inlining all submodule and functor definitions in their long form is not always the optimal choice.
In particular, it can be better to be more concise and use only the short description for nested submodules

This PR adds a flag --shorten-beyond-depth to the latex backend to make it choose the abbreviated form
for submodules beyond a given depth.

Along the way, this PR fixes a collision between page url and anchor and adds missing label definitions for pages that have been inlined in the current page in order to redirect references to those page toward the inlined contents.

Combined with an option to remove link to functor arguments, this seems enough to remove all missing labels warning from the build of the reference manual.

[Octachron]

@jonludlam , would you prefer to me to add a Changes entry with the initial PR?

By default, I tend to add it once the PR has been reviewed, but switching to an optimist mode for the Changes entry is fine with me.

[jonludlam]

I don't particularly mind when it's added - but since this has been languishing for ages without being looked at, I thought I wouldn't then send it back because it didn't have CHANGES :-) Sorry it's been so long! It looks fine though, so as soon as the tests are all passing, in it goes!