Ensured that maximum and minimum column length assertions are only asserted where actually relevant.

Rewrote provideColumnLengths() to return fixed rather than variable-length result.

- Improved descriptive output. ...

- Added sqlifyValue function to improve flexibility of SQL string handling.
- Renamed 'type' property to 'sql_type'.
- Added 'generic_type' property.
- Improved testing of legal and illegal values.

Added preliminary output based on whether an illegal value is rejected by a constraint or an actual overflow.

- Slightly tidier printing of messages involving plurals. ...

- Now properly handles both "check constraint violated" and "value larger than specified precision" errors.

Added experimental unit testing code.