This important commit introduces the 'DbCmd' constraint set, allowing DB
functions to require only a connection pool and some `GargConfig`.

It's not fully wired in, `convertQuery` for Arxiv is implemented as
`undefined`.

Also, added a basic GraphQL endpoint.

This commit switches to an Hackage-released version of HSvm,
specifically 0.1.1.3.22, which is the same we were previously
relying on, but by pinning on a specific commit
`3fe28b683aba5ddf05e3b5f8eced0bd05c5a29f9` of the GitHub repo.

This also restores commit `243cfbb5`,
which got misteriously missing.

This commit removes the `servant-static-th` dependency from the project,
because it turns out we weren't using this package, and furthemore we
were depending on a fork of the latter.

We were depending on Alexandre's fork of `hstatistics`, but in the
meantime his changes got merged upstream and the maintainer release
version `0.3.1` on Hackage, so this commit changes the `stack.yaml` file
to use that.

Remove joins, use optionalRestrict instead.

Also, remove cabal.project because stack.yaml and package.yaml cover
dependencies already.