s2: Use sorted search for index (#555)

```
cpu: AMD Ryzen 9 3950X 16-Core Processor
BenchmarkIndexFind
BenchmarkIndexFind/blocks-1-32         	194769370	         6.180 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2-32         	199258790	         6.057 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4-32         	204297220	         5.879 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8-32         	190476280	         6.321 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16-32        	158611744	         7.537 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32-32        	100000000	        10.04 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-64-32        	79986668	        14.90 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-128-32       	74996250	        14.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-256-32       	19506928	        60.93 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-512-32       	 8450894	       142.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-1024-32      	 3858536	       307.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2048-32      	 2556153	       470.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4096-32      	  840476	      1457 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8192-32      	  571398	      2091 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16384-32     	  195525	      6022 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32768-32     	  138596	      8665 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-65535-32     	   86328	     13993 ns/op	       0 B/op	       0 allocs/op
```

after:

```
cpu: AMD Ryzen 9 3950X 16-Core Processor
BenchmarkIndexFind
BenchmarkIndexFind/blocks-1-32         	195354979	         6.054 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2-32         	193981434	         6.140 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4-32         	201894782	         6.023 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8-32         	180996812	         6.978 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16-32        	148147708	         8.016 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32-32        	100000000	        10.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-64-32        	75000000	        15.95 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-128-32       	75000000	        15.59 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-256-32       	49979174	        22.83 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-512-32       	46302012	        25.00 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-1024-32      	44444937	        27.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-2048-32      	38709552	        29.62 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-4096-32      	37005741	        31.86 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-8192-32      	34297766	        34.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-16384-32     	30763077	        36.89 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-32768-32     	29656209	        39.45 ns/op	       0 B/op	       0 allocs/op
BenchmarkIndexFind/blocks-65535-32     	27980180	        42.53 ns/op	       0 B/op	       0 allocs/op
```

Fixes #554

s2/encode_test.go

@@ -351,6 +351,43 @@ func TestIndex(t *testing.T) {
 	}
 }
 
+func BenchmarkIndexFind(b *testing.B) {
+	fatalErr := func(t testing.TB, err error) {
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+	for blocks := 1; blocks <= 65536; blocks *= 2 {
+		if blocks == 65536 {
+			blocks = 65535
+		}
+
+		var index Index
+		index.reset(100)
+		index.TotalUncompressed = int64(blocks) * 100
+		index.TotalCompressed = int64(blocks) * 100
+		for i := 0; i < blocks; i++ {
+			err := index.add(int64(i*100), int64(i*100))
+			fatalErr(b, err)
+		}
+
+		rng := rand.New(rand.NewSource(0xabeefcafe))
+		b.Run(fmt.Sprintf("blocks-%d", len(index.info)), func(b *testing.B) {
+			b.ResetTimer()
+			b.ReportAllocs()
+			const prime4bytes = 2654435761
+			rng2 := rng.Int63()
+			for i := 0; i < b.N; i++ {
+				rng2 = ((rng2 + prime4bytes) * prime4bytes) >> 32
+				// Find offset:
+				_, _, err := index.Find(rng2 % (int64(blocks) * 100))
+				fatalErr(b, err)
+			}
+		})
+	}
+
+}
+
 func TestWriterPadding(t *testing.T) {
 	n := 100
 	if testing.Short() {

s2/index.go

@@ -10,6 +10,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"sort"
 )
 
 const (
@@ -100,6 +101,15 @@ func (i *Index) Find(offset int64) (compressedOff, uncompressedOff int64, err er
 	if offset > i.TotalUncompressed {
 		return 0, 0, io.ErrUnexpectedEOF
 	}
+	if len(i.info) > 200 {
+		n := sort.Search(len(i.info), func(n int) bool {
+			return i.info[n].uncompressedOffset > offset
+		})
+		if n == 0 {
+			n = 1
+		}
+		return i.info[n-1].compressedOffset, i.info[n-1].uncompressedOffset, nil
+	}
 	for _, info := range i.info {
 		if info.uncompressedOffset > offset {
 			break