Three test failures remain, but these are minor and will be resolved
soon. Handling of binary data is also broken, but given that this works
fine with the PDO driver, there is presumably some correct method.
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
