Tidy up many open-coded trait impls

• Replace many open-coded Default with #[derive]. I chose not to replace all that could be replaced; in general, I avoided doing so when it would involve implicitly assigning zero to numeric parameters.
• Replace one unnecessary open-coded Clone impl.
• Where Default called new, but the functionality was sensibly available via #[derive(Default)], swap the call over so that new calls Default, and use the derive.
• Some minor debug improvements.

These changes are ones I came across while working on #366 (closed), for which I did a grep for manual impls of Clone, Debug, and Default.