This commit adds alternative implementations of methods of ImmSet:
 * InsertNew(xs ...T)
 * DeleteNew(xs ...T)
 * UnionNew(s2 ImmSet[T])
 * DifferenceNew(s2 ImmSet[T])
and benchmarks these implementations agains the existing ones.

Benchmarking results:
 * for Insert, the proposed method becomes faster already with the
   container of size 1000, and then it performed 10x faster for size
   10,000 and 100x faster for size 100,000;
 * for Delete, the proposed method becomes faster already with the
   container of size 1000, and then it performed ~5x faster for size
   10,000;
 * for Difference, the proposed method was already 4x faster for size
   100, and then it performed 7x faster for size 1000, 35x times faster
   for size 10,000, and 193x faster for size 100,000;
 * for Union, the proposed method performs slightly faster, but gains
   do not visibly grow with increasing size.

Theoretically, the proposed solutions have improved computational
complexity:
 * the complexity of Insert is O(len(s.xs)*len(xs)), and the complexity
   of InsertNew is O(len(s.xs)+len(xs));
 * the complexity of Delete is O(len(s.xs)*len(xs)), and the complexity
   of DeleteNew is O(len(s.xs)+len(xs));
 * the complexity of Difference is O(len(s.xs)*len(s2.xs)) because it
   uses Delete internally, and the complexity of DifferenceNew
   O(len(s.xs)+len(s2.xs));
 * the complexity of Union is harder to estimate: it involves sorting a
   slice of size n=len(s.xs)+len(s2.xs), but this slice is a
   concatenation of two sorted slices, so most likely this does not lead
   to the usual O(n*log(n)) complexity; of course, it is at least O(n);
   the complexity of UnionNew is O(n).

Signed-off-by: Damian Sawicki <dsawicki@google.com>