groot/rcmd: improve Dump tree performances

```
  name    old time/op    new time/op    delta
  Dump-8     5.02s ± 2%     3.84s ± 2%  -23.57%  (p=0.000 n=29+27)

  name    old alloc/op   new alloc/op   delta
  Dump-8    2.50GB ± 0%    2.40GB ± 0%   -3.82%  (p=0.000 n=29+30)

  name    old allocs/op  new allocs/op  delta
  Dump-8     38.8M ± 0%     29.3M ± 0%  -24.69%  (p=0.000 n=30+30)
```

groot/rcmd/dump.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"reflect"
+	"strconv"
 
 	"go-hep.org/x/hep/groot"
 	"go-hep.org/x/hep/groot/rdict"
@@ -133,14 +134,43 @@ func (cmd *dumpCmd) dumpTree(t rtree.Tree) error {
 	}
 	defer r.Close()
 
+	names := make([][]byte, len(vars))
+	for i, v := range vars {
+		name := v.Name
+		if v.Leaf != "" && v.Leaf != v.Name {
+			name = v.Name + "." + v.Leaf
+		}
+		names[i] = []byte(name)
+	}
+
+	// FIXME(sbinet): don't use a "global" buffer for when rtree.Reader reads multiple
+	// events in parallel.
+	buf := make([]byte, 0, 8*1024)
+	hdr := make([]byte, 0, 6)
 	err = r.Read(func(rctx rtree.RCtx) error {
-		for _, v := range vars {
-			name := v.Name
-			if v.Leaf != "" && v.Leaf != v.Name {
-				name = v.Name + "." + v.Leaf
-			}
+		hdr = hdr[:0]
+		hdr = append(hdr, '[')
+		switch {
+		case rctx.Entry < 10:
+			hdr = append(hdr, '0', '0')
+		case rctx.Entry < 100:
+			hdr = append(hdr, '0')
+		}
+		hdr = strconv.AppendInt(hdr, rctx.Entry, 10)
+		hdr = append(hdr, ']', '[')
+		for i, v := range vars {
+			buf = buf[:0]
+			buf = append(buf, hdr...)
+			buf = append(buf, names[i]...)
+			buf = append(buf, ']', ':', ' ')
 			rv := reflect.Indirect(reflect.ValueOf(v.Value))
-			fmt.Fprintf(cmd.w, "[%03d][%s]: %v\n", rctx.Entry, name, rv.Interface())
+			// All of this is a convoluted (but efficient) way to do:
+			//  fmt.Fprintf(cmd.w, "[%03d][%s]: %v\n", rctx.Entry, name, rv.Interface())
+			buf = append(buf, fmt.Sprintf("%v\n", rv.Interface())...)
+			_, err = cmd.w.Write(buf)
+			if err != nil {
+				return err
+			}
 		}
 		return nil
 	})

groot/rcmd/dump_test.go

@@ -343,3 +343,18 @@ key[004]: tree;1 "my tree title" (TTree)
 		})
 	}
 }
+
+func BenchmarkDump(b *testing.B) {
+	const deep = true
+	out := new(strings.Builder)
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		out.Reset()
+		b.StartTimer()
+		// big-file.root is: rtests.XrdRemote("testdata/SMHiggsToZZTo4L.root")
+		err := rcmd.Dump(out, "../testdata/big-file.root", deep, nil)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}