perf: reduce allocation in serializing for frequency sketch

[proost]

During serialization, current implementation allocates same active items multiple times. It makes huge overhead because of heap allocation.

See the benchmark:

Current main branch:

goos: darwin
goarch: arm64
pkg: github.com/apache/datasketches-go/frequencies
cpu: Apple M1 Pro
BenchmarkItemsSketchToSlice
BenchmarkItemsSketchToSlice/int64/items=48
BenchmarkItemsSketchToSlice/int64/items=48-10         	  208099	      6027 ns/op	 132.73 MB/s	   20096 B/op	      51 allocs/op
BenchmarkItemsSketchToSlice/int64/items=192
BenchmarkItemsSketchToSlice/int64/items=192-10        	   14924	     80697 ns/op	  38.46 MB/s	  301185 B/op	     195 allocs/op
BenchmarkItemsSketchToSlice/int64/items=768
BenchmarkItemsSketchToSlice/int64/items=768-10        	    1072	   1082348 ns/op	  11.38 MB/s	 4744468 B/op	     771 allocs/op
BenchmarkItemsSketchToSlice/string/items=48
BenchmarkItemsSketchToSlice/string/items=48-10        	  200163	      5586 ns/op	 167.22 MB/s	   20928 B/op	      51 allocs/op
BenchmarkItemsSketchToSlice/string/items=192
BenchmarkItemsSketchToSlice/string/items=192-10       	   15355	     75696 ns/op	  49.70 MB/s	  304512 B/op	     195 allocs/op
BenchmarkItemsSketchToSlice/string/items=768
BenchmarkItemsSketchToSlice/string/items=768-10       	    1129	   1072233 ns/op	  14.25 MB/s	 4758036 B/op	     771 allocs/op

Change:

goos: darwin
goarch: arm64
pkg: github.com/apache/datasketches-go/frequencies
cpu: Apple M1 Pro
BenchmarkItemsSketchToSlice
BenchmarkItemsSketchToSlice/int64/items=48
BenchmarkItemsSketchToSlice/int64/items=48-10         	 2943316	       401.4 ns/op	1992.87 MB/s	    1664 B/op	       3 allocs/op
BenchmarkItemsSketchToSlice/int64/items=192
BenchmarkItemsSketchToSlice/int64/items=192-10        	  865233	      1399 ns/op	2218.19 MB/s	    6272 B/op	       3 allocs/op
BenchmarkItemsSketchToSlice/int64/items=768
BenchmarkItemsSketchToSlice/int64/items=768-10        	  211227	      5016 ns/op	2456.31 MB/s	   25856 B/op	       3 allocs/op
BenchmarkItemsSketchToSlice/string/items=48
BenchmarkItemsSketchToSlice/string/items=48-10        	 2000376	       599.6 ns/op	1557.73 MB/s	    2496 B/op	       3 allocs/op
BenchmarkItemsSketchToSlice/string/items=192
BenchmarkItemsSketchToSlice/string/items=192-10       	  575967	      2041 ns/op	1843.51 MB/s	    9600 B/op	       3 allocs/op
BenchmarkItemsSketchToSlice/string/items=768
BenchmarkItemsSketchToSlice/string/items=768-10       	  145569	      9110 ns/op	1677.53 MB/s	   39424 B/op	       3 allocs/op
PASS