Implement Stringer interface for boil-defined types in types package

[xfzhu01]

Background
In some scenarios, I want to print models that boiler generated to stdout using fmt package for debug, But sometimes it throws panic because there are nil fields for some boil-defined type, such as types.NullDecimal, The root cause is those types do not implement Stringer interface.
Solution
I think it's good practice to implement Stringer interface for boil-defined types. e.g. For types.NullDecimal

func (d *NullDecimal) String() string {
	if d == nil || d.Big == nil {
		return "<nil>"
	}
	return d.Big.String()
}

Does it make sense?

[aarondl]

Try using a better debug output library like go-spew. It will correctly handle introspecting the nils and will give you much better output anyway.