Don't automatically freeze the results of RowSegment ops

I think when this code was written, I thought "freeze" would be
really cheap. It's not actually that cheap. As a result, freezing
things preemptively when it may be that nothing ever tries to write
to them anyway is possibly disadvantageous, to the tune of being
roughly 20% of a sample profile we were shown. Instead, we don't
mark the components "writable", so if anything wants to write to
them, it'll end up freezing itself new copies of their bitmaps
later. But in practice that probably doesn't happen.

row.go

@@ -574,13 +574,11 @@ func (s *rowSegment) IntersectionCount(other *rowSegment) uint64 {
 // Intersect returns the itersection of s and other.
 func (s *rowSegment) Intersect(other *rowSegment) *rowSegment {
 	data := s.data.Intersect(other.data)
-	data = data.Freeze()
 
 	return &rowSegment{
-		data:     data,
-		shard:    s.shard,
-		n:        data.Count(),
-		writable: true,
+		data:  data,
+		shard: s.shard,
+		n:     data.Count(),
 	}
 }
 
@@ -591,13 +589,11 @@ func (s *rowSegment) Union(others ...*rowSegment) *rowSegment {
 		datas[i] = other.data
 	}
 	data := s.data.Union(datas...)
-	data.Freeze()
 
 	return &rowSegment{
-		data:     data,
-		shard:    s.shard,
-		n:        data.Count(),
-		writable: true,
+		data:  data,
+		shard: s.shard,
+		n:     data.Count(),
 	}
 }
 
@@ -635,26 +631,22 @@ func (s *rowSegment) Difference(others ...*rowSegment) *rowSegment {
 		datas[i] = other.data
 	}
 	data := s.data.Difference(datas...)
-	data.Freeze()
 
 	return &rowSegment{
-		data:     data,
-		shard:    s.shard,
-		n:        data.Count(),
-		writable: true,
+		data:  data,
+		shard: s.shard,
+		n:     data.Count(),
 	}
 }
 
 // Xor returns the xor of s and other.
 func (s *rowSegment) Xor(other *rowSegment) *rowSegment {
 	data := s.data.Xor(other.data)
-	data = data.Freeze()
 
 	return &rowSegment{
-		data:     data,
-		shard:    s.shard,
-		n:        data.Count(),
-		writable: true,
+		data:  data,
+		shard: s.shard,
+		n:     data.Count(),
 	}
 }
 
@@ -666,13 +658,11 @@ func (s *rowSegment) Shift() (*rowSegment, error) {
 	if err != nil {
 		return nil, errors.Wrap(err, "shifting roaring data")
 	}
-	data = data.Freeze()
 
 	return &rowSegment{
-		data:     data,
-		shard:    s.shard,
-		n:        data.Count(),
-		writable: true,
+		data:  data,
+		shard: s.shard,
+		n:     data.Count(),
 	}, nil
 }