BitArray from int array and binary

[l0rinc]

Since we can export easily to an int array, we should be able to import it as well, i.e.:

  var bits = BitArray(history.keys.max)..setBits(setKeys); // would be nice to have a direct constructor for this as `BitArray(setKeys)`
  var ints = bits.asUint32Iterable().toList();

  expect(BitArray.parseInts(ints), bits);

This should be better optimized than #4.
Even better solution could be to serialize/deserialize from binary string representation - I need this BitArray for optimized storage in the first place.

[isoos]

I think this is a good idea, feel free to create a PR for it.