IF NOT EXISTS makes sense only when you provide the name of the index.

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

Merge remote-tracking branch 'origin/239-readme-test-reinstall-project-with-cabal-and-update-the-readme-file-2' into dev

This is to prevent accidental updates of hyperdata with other JSON
structs.

The bug was introduced in

!177 (diffs)

It caused hyperdata to be updated with whole corpus, serialized, hence
hyperdata inside hyperdata.