Add short types: nullable_t and non_nullable_t

As discussed before, here is my contribution. I've modified a little the type which is now two types: one for nullable and the other for non_nullable.

You can see the use case here: https://bitbucket.org/Engil/cumulus/src/d862fcac45f987c798a039dfc64a0c969c8caa7f/src/db_user.mli?at=master

I think it's more clear and short than having an object in an other object.