* Support multiple statements for mysql

* Make multiple statements a mysql connection option

- improve code style

The types here are getting a little confusing,
in particular the implied contracts around when the result
array needs to be nonempty. Eventually I will rework these types
to properly encode the appropriate conditions and enforce everything
at compile-time.

Change driver query interface to always return tabular output.
In cases where exec queries return metadata, force driver
implementations to normalize the response into a clean tabular type.