Binary Indexed Trees [C++]

#### This issue is about [Binary Indexed Trees](https://www.topcoder.com/community/data-science/data-science-tutorials/binary-indexed-trees).

- [x] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [x] Assign this issue to me (I want to work on this).
- [x] I ensure that I am not already assigned to 2 or more issues.
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Start with the linked Topcoder Binary Indexed Trees tutorial and inspect the repository for existing algorithm and data-structure implementations in C++. Add the Binary Indexed Tree implementation in the repository's established style; done means the requested C++ algorithm is represented consistently with the other entries.

Written by the indexing model from the issue text.