[proost]

We can reduce serialization twice to once.

benchmark code:

func BenchmarkGetSerializedSizeBytes(b *testing.B) {
	comparator := common.ItemSketchStringComparator(false)
	serde := common.ItemSketchStringSerDe{}
	sketch, _ := NewKllItemsSketch[string](_DEFAULT_K, _DEFAULT_M, comparator, serde)
	for i := 0; i < 10000; i++ {
		sketch.Update(fmt.Sprintf("%05d", i))
	}
	b.ReportAllocs()
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		sketch.GetSerializedSizeBytes()
	}
}

AS-IS:

goos: darwin
goarch: arm64
pkg: github.com/apache/datasketches-go/kll
cpu: Apple M4 Pro
BenchmarkGetSerializedSizeBytes
BenchmarkGetSerializedSizeBytes-12    	       1	1372407375 ns/op	   32224 B/op	     542 allocs/op
PASS

TO-BE:

goos: darwin
goarch: arm64
pkg: github.com/apache/datasketches-go/kll
cpu: Apple M4 Pro
BenchmarkGetSerializedSizeBytes
BenchmarkGetSerializedSizeBytes-12    	 1000000	      1003 ns/op	       0 B/op	       0 allocs/op
PASS