Enhance `LazySegtree` `Debug` implementation

[NotLeonian]

Resolves #143

This draft PR implements the Debug trait for LazySegtree. As mentioned in the issue, the Debug implementation uses the debug_struct function to display the relevant fields. I would mainly like to confirm the following points:

• Whether the proposed output format is acceptable,
• Whether generating a 2-dimensional Vec to display d and lz as lists is reasonable,
• And whether using a method chain for this implementation is appropriate.