Finger Trees

Implement the Finger Trees data structure.

Reference: [http://www.staff.city.ac.uk/~ross/papers/FingerTree.html](http://www.staff.city.ac.uk/~ross/papers/FingerTree.html).

The issue references a research paper on Finger Trees. Start by reading the linked paper to understand the data structure. Look at existing data structure implementations in the repository for patterns. Implement the core Finger Tree operations (like prepend, append, split, concat) and ensure they are generic. Write unit tests to verify correctness.

索引モデルが issue の本文から書いたものです。