Reviewed visibility

[daladim]

This PR depends on #63 , #65 , #66 and #68 (it does not depend on all of them feature-wise, but merging them first will avoid git conflicts, especially in the use ....; lines at the beginning of files)

This PR (or rather, its last 5 commits) reviews the visibility of many types and modules, so that the user-facing API and documentation only exposes what he will need.

• This makes it easier to read the doc
• This leaves us more room to do future changes that otherwise could be breaking (since breaking change in a public API requires a new major version. That's too bad for types that are not supposed to be used).

Maybe(?) some issues were pre-existing, but I think most of these too-widly-visible items were due to uncautious refactos of mine

[daladim]

Build is failing. I probably did not include all the dependant commits after all. I'll rebase after other PR (the ones that are passing) are merged, that will be easier to rebase.

You can consider this PR as a draft for the moment

[daladim]

I've rebased so that it now compiles and passes the tests.

I've also added a commit that removes unused error types.