~ Use IntSet instead of ByteString for bitfields.

There are several reasons for this:

* IntSet is stored in ordinary heap, while ByteStrings in pinned memory;
* Our IntSet's should be much faster 90% time. (in typical BT client)
  Hovewer in worst case IntSet is slower, but difference should is not
  so big. (We should measure this although)
* It's pure, tested, error-free and much more convenient.

Moreover we have kill a lot of ugly code!