generic finger-tree structure (http://staff.city.ac.uk/~ross/papers/FingerTree.html)
#18NFData instance for FingerTree
Hi! First of all, I love this library and also the paper.
That being said, we recently tried to use this (over hw-fingertree) and encountered the shortcoming that it didn't provide an NFData instance. I wonder if this is because you don't want it to have any dependencies or just because there's some different kind of reason.
I would be happy to contribute this change if you'd not have any issue with it :)
