Add compatibility for nim 2.0
I know you're already aware of this, but I believe making it an issue would still be useful to document our trail of thoughts as well as progress.

With nim 2.0 the packages std/db_sqlite/db_postgres/db_mysql are no longer part of the standard library!

They have been moved into its own package called [dbconnector](https://github.com/nim-lang/db_connector).
I do not believe this affects norm directly as I'm not aware of any std/db_* imports there, but it does affect norm's ndb dependency.
It makes use of std/sqlite3 and stb/db_common.

Over there it should be as simple as swapping out the imports:

    std/sqlite3 => db_connector/sqlite3
    std/db_common => db_connector/db_common

Once this is fixed, it will require an update of the ndb dependency.

Now the question is what to do about this?
ndb appears to not be receiving any further work.
Should we fork?
Or PR to the original ndb upstream?
Or remove the ndb dependency entirely and move that code into norm?

The maintainer of ndb does appear to still accept PRs, so working with that repo seems still possible.
I personally would prefer not to move ndb code into norm, as the value of a postgres/sqlite wrapper that allows for more correctness is something not just norm can make use of.