Simplify Statement type
Be consistent with whitespace use in the rest of the module

+ Remove trailing whitespace
Default to 'no query string' if hitting NULL pointer with sqlite3_sql

This case should never happen, so avoid introducing a new exception
type for this.
Use "ToRow p => p" constraint for bind instead of a list of SQLDatas

withBind and bind should have the same types for query parameters.
Constrain 'bind' with the ToRow class and convert input parameters to
SQLDatas internally.
Add SQL statement debug tracing (setTrace)

Fixes #21