Implement Clone and Debug for FenwickTree

[NotLeonian]

Resolves #145

This draft PR implements the Clone and Debug traits for FenwickTree.
The Clone trait is implemented using derive.

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.