[VIndex] Added benchmark for indexing 10k entries (#129)

This gives a target when switching over the MPT implementation.

```
go test -bench=BenchmarkBuild -benchmem ./vindex

goos: linux
goarch: amd64
pkg: github.com/transparency-dev/incubator/vindex
cpu: AMD EPYC 7B12
BenchmarkBuild_InMemory-24             1        1004559670 ns/op        38192304 B/op     588597 allocs/op
BenchmarkBuild_OnDisk-24               1        4810209571 ns/op        2268211632 B/op  1541229 allocs/op
PASS
ok      github.com/transparency-dev/incubator/vindex    26.039s
```

Author: mhutchinson

vindex/map_test.go

@@ -22,6 +22,7 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"iter"
+	"math/rand"
 	"os"
 	"path"
 	"sync"
@@ -43,6 +44,9 @@ import (
 const (
 	skey = "PRIVATE+KEY+logandmap+38581672+AXJ0FKWOcO2ch6WC8kP705Ed3Gxu7pVtZLhfHAQwp+FE"
 	vkey = "logandmap+38581672+Ab/PCr1eCclRPRMBqw/r5An1xO71MCnImLiospEq6b4l"
+
+	benchNumEntries       = 10000
+	benchDuplicationRatio = 0.5
 )
 
 func TestVerifiableIndex(t *testing.T) {
@@ -338,3 +342,86 @@ func (s *inMemoryTreeSource) Append(leafStr string) {
 	s.leaves = append(s.leaves, leaf)
 	s.t.Append(rfc6962.DefaultHasher.HashLeaf(leaf))
 }
+
+func runBenchmark(b *testing.B, opts vindex.Options) {
+	ctx := context.Background()
+	s, v, err := fnote.NewEd25519SignerVerifier(skey)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	inputLog := &inMemoryTreeSource{
+		t:      testonly.New(rfc6962.DefaultHasher),
+		leaves: make([][]byte, 0),
+		s:      s,
+		v:      v,
+	}
+
+	rng := rand.New(rand.NewSource(12345))
+
+	var uniqueKeys []string
+	for i := range benchNumEntries {
+		var key string
+		if len(uniqueKeys) > 0 && rng.Float64() < benchDuplicationRatio {
+			key = uniqueKeys[rng.Intn(len(uniqueKeys))]
+		} else {
+			key = fmt.Sprintf("key-%d", len(uniqueKeys))
+			uniqueKeys = append(uniqueKeys, key)
+		}
+		inputLog.Append(fmt.Sprintf("%s: %d", key, i))
+	}
+
+	mapFn := func(leaf []byte) [][sha256.Size]byte {
+		k, _, found := bytes.Cut(leaf, []byte(":"))
+		if !found {
+			panic("colon not found")
+		}
+		return [][sha256.Size]byte{sha256.Sum256(k)}
+	}
+
+	b.ResetTimer()
+	b.ReportAllocs()
+
+	for b.Loop() {
+		b.StopTimer()
+		dir, err := os.MkdirTemp("", "vindex-bench")
+		if err != nil {
+			b.Fatal(err)
+		}
+
+		iterCtx, iterCancel := context.WithCancel(ctx)
+
+		outputLog, closer, err := vindex.NewOutputLog(iterCtx, path.Join(dir, "outputlog"), s, v, vindex.OutputLogOpts{})
+		b.Cleanup(func() {
+			iterCancel()
+			_ = os.RemoveAll(dir)
+			if closer != nil {
+				closer()
+			}
+		})
+		if err != nil {
+			b.Fatal(err)
+		}
+
+		vi, err := vindex.NewVerifiableIndex(iterCtx, inputLog, mapFn, outputLog, dir, opts)
+		if err != nil {
+			b.Fatal(err)
+		}
+
+		b.StartTimer()
+		if err := vi.Update(iterCtx); err != nil {
+			b.Fatal(err)
+		}
+		if err := vi.Close(); err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkBuild_InMemory(b *testing.B) {
+	runBenchmark(b, vindex.Options{PersistIndex: false})
+}
+
+func BenchmarkBuild_OnDisk(b *testing.B) {
+	runBenchmark(b, vindex.Options{PersistIndex: true})
+}