This finally brings PostgreSQL to parity with SQLite and MySQL.
Two tests casting binary data to text were removed since behaviour here
should in fact be undefined
Accountinf for any encoding when retrieving data will be addressed by
a later commit
No testing has been performed yet, but changes are extensive enough to
warrant a commit. Of particular note:
- SQL states are enumerated in a separate trait to reduce duplication
- PDOStatement is now an abstract class to avoid duplication of
engine-specific error handling
- Error handling has been cleaned up somewhat
