This commit makes the following changes
- Add an opaque `Debug` impl for `Client`.
- Add a rich `Debug` impl for `Row`.
- Make the `Debug` impl for `Type` clearer.
- Change the `Debug` for `Column` to be slightly neater.
Benchmarks indicate that malloc accounts for a significant amount of the
runtime of queries. The message buffer accounts for ~half of that (the
other being channels), and this change should eliminate it.
