refactor: lift type mappings into driver crates

[abonander]

Motivated by #2917

cc @g-bartoszek you can build your changes onto this. Just use Postgres::fmt_value_debug()

[abonander]

While I doubt anyone's actually using the type mappings I had to change (to actually get this to compile), it's still technically a breaking change and will need to wait for the next major release.

On the upside, it's possible to catch mistakes like this in the future because the macro invocation will complain about the lack of a Decode or Encode impl.