Implement the Clone and Debug traits for FenwickTree.

For the Clone trait, it should be enough to add a Clone trait bound to T and write an explicit impl.

For the Debug trait, I think that discussion is needed regarding the output format.
Unlike Segtree and LazySegtree, FenwickTree does not have self.log.
Additionally, in a Fenwick Tree, the structure does not explicitly store all segments like a Segment Tree or Lazy Segment Tree. This difference causes gaps in the tree representation, making it problematic to use the same output format.

Therefore, I propose an output format that includes self.n, self.e, and the return values of the accum function for 1..=self.n.
For example, if self.n = 10, self.e = 0, and the array is [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], the output could be one of the following:

FenwickTree { n: 10, e: 0, accum: [0, 1, 3, 6, 10, 15, 21, 28, 36, 45] }

n: 10
e: 0
accum: 0        1       3       6       10      15      21      28      36      45

With this implementation, the required trait bounds for T are limited to Clone + Debug + std::ops::AddAssign<T>.