benchmark: add a comparison of 'archive/tar'

Since this project has forked logic of upstream 'archive/tar', this does
a brief comparison including the RawBytes usage.

```bash
$ go test -run="XXX" -bench=.
testing: warning: no tests to run
BenchmarkUpstreamTar-4                      2000            700809 ns/op
BenchmarkOurTarNoAccounting-4               2000            692055 ns/op
BenchmarkOurTarYesAccounting-4              2000            723184 ns/op
PASS
ok      vb/tar-split    4.461s
```

From this, the difference is negligible.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

tar_benchmark_test.go

@@ -0,0 +1,84 @@
+package tartest
+
+import (
+	"io"
+	"io/ioutil"
+	"os"
+	"testing"
+
+	upTar "archive/tar"
+
+	ourTar "github.com/vbatts/tar-split/archive/tar"
+)
+
+var testfile = "./archive/tar/testdata/sparse-formats.tar"
+
+func BenchmarkUpstreamTar(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		fh, err := os.Open(testfile)
+		if err != nil {
+			b.Fatal(err)
+		}
+		tr := upTar.NewReader(fh)
+		for {
+			_, err := tr.Next()
+			if err != nil {
+				if err == io.EOF {
+					break
+				}
+				fh.Close()
+				b.Fatal(err)
+			}
+			io.Copy(ioutil.Discard, tr)
+		}
+		fh.Close()
+	}
+}
+
+func BenchmarkOurTarNoAccounting(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		fh, err := os.Open(testfile)
+		if err != nil {
+			b.Fatal(err)
+		}
+		tr := ourTar.NewReader(fh)
+		tr.RawAccounting = false // this is default, but explicit here
+		for {
+			_, err := tr.Next()
+			if err != nil {
+				if err == io.EOF {
+					break
+				}
+				fh.Close()
+				b.Fatal(err)
+			}
+			io.Copy(ioutil.Discard, tr)
+		}
+		fh.Close()
+	}
+}
+func BenchmarkOurTarYesAccounting(b *testing.B) {
+	for n := 0; n < b.N; n++ {
+		fh, err := os.Open(testfile)
+		if err != nil {
+			b.Fatal(err)
+		}
+		tr := ourTar.NewReader(fh)
+		tr.RawAccounting = true // This enables mechanics for collecting raw bytes
+		for {
+			_ = tr.RawBytes()
+			_, err := tr.Next()
+			_ = tr.RawBytes()
+			if err != nil {
+				if err == io.EOF {
+					break
+				}
+				fh.Close()
+				b.Fatal(err)
+			}
+			io.Copy(ioutil.Discard, tr)
+			_ = tr.RawBytes()
+		}
+		fh.Close()
+	}
+}