Improve DataObject.save() generation

[jayvdb]

• Use Self:: and self. more to access existing accessible values, so generated code is easier to read. i.e. less duplicate literals in the output.
  • I am assuming the compiler knows <Self as butane::DataResult>::COLUMNS.len() is a constant.
• Removes an if values.len() > 0 from the .save() for tables with an AutoPk column that has already been inserted.
  • no doubt the compiler can detect whether values is or is not empty, but a human needs to think about it.
• Also trim the generated code for a table with only one column which is AutoPk, and add a test case for this.