Include Prelude as submodule

[f-f]

Right now we have two distinct copies of Prelude: one in this repo and another in its own repo.
Since new developments happen on the dedicated repo, the two copies will get out of sync (this has happened already)

To prevent syncing issues we should include that repo as a submodule, but Hydra doesn't like that at the moment, as seen here

[Gabriella439]

Fixed in #639 by using Dhall's import system to retrieve the Prelude