BitSet fixes

* `LongBitSet.ensureCapacity` overflows on `numBits > Integer.MaxValue`
* `Fixed-/LongBitSet`: Avoid conditional branch in `bits2words` (with a comment explaining the formula)

TODO:
* Harmonize the use of `numWords` vs. `bits.length` vs. `numBits`
 * E.g.: `cardinality` scans up to `bits.length`, while `or` asserts on `index<numBits`
* If a `BitSet` is allocated with `n` bits, `ensureCapacity` with the same number `n` shouldn't grow the `BitSet`
 * Either both should allocate a larger array than really needed or neither.