Make the modules pub again and write module documents

[qryxip]

In #6, we decided to make all of the modules crate-private.

But maybe we should make the mods pub again and write doc comments in each modules, as in the original ACL they split the description into modules.

Moreover, our ACL provides extra types/traits. They are unlikely used directly and they should be public (i.e. reachable). For example, user should be able to see modint::RemEuclidU32 is implemented for all of the primitive integers. They should be in each modules keeping their documents reachable.