Support for h_store

[ghivert]

Postgres supports the h_store type, to manage a hash map directly in PG columns. While it's probably easier to use jsonb nowadays, it could be useful to add support for older DB.

After diving in pg_types, used internally by pgo, I figured out that pushing a native gleam Dict is enough. Right now, using the ugly dynamic trick (from |> unsafe_coerce) is working, but we could think to add a function like hstore: fn (Dict(String, a)) -> Value in the future.

I'm not sure we should add such a function to the package, but I think it's useful enough to keep this info somewhere, at least to let people interested to know there's a solution to their problem, and wait before someone really needs it before implementing a solution? To make sure we have a correct API.