FileUtils.sizeOfDirectoryAsBigInteger can overflow

FileUtils.sizeOfDirectoryAsBigInteger can overflow.

This is because it calls FileUtils.sizeOf(file) which calls sizeOfDirectory() for processing subdirectories.

As it stands, the method only works properly at the top level.

A possible solution would be to create private "Big" versions of the called methods; these would not need all the checks so should be faster.