Implement `Debug` for `FenwickTree`

[NotLeonian]

Resolves #145

This draft PR implements the Debug trait for FenwickTree. The previous draft PR with the same content was unintentionally closed, so I’ve created a new branch to both reopen it and clean up the commit history. Below is the same explanation as before, for reference.

As mentioned in the issue, the Debug implementation uses the debug_struct function to display the fields n, accum, and e. I would mainly like to confirm the following points:

• Whether the proposed output format is acceptable,
• Whether generating a Vec to display accum as a list is reasonable,
• And whether using a method chain for this implementation is appropriate.