Add binary decoding benchmarks

```
BenchmarkBinaryDecodingManual-4         10479085               106 ns/op              40 B/op          2 allocs/op
BenchmarkBinaryDecodingHelpers-4         4485451               263 ns/op              64 B/op          4 allocs/op
BenchmarkBinaryDecodingRow-4             1999726               587 ns/op              96 B/op          5 allocs/op
```

composite_bench_test.go

@@ -5,6 +5,7 @@ import (
 
 	"github.com/jackc/pgtype"
 	"github.com/jackc/pgtype/binary"
+	errors "golang.org/x/xerrors"
 )
 
 type MyCompositeRaw struct {
@@ -30,6 +31,45 @@ func (src MyCompositeRaw) EncodeBinary(ci *pgtype.ConnInfo, buf []byte) (newBuf
 	return
 }
 
+func (dst *MyCompositeRaw) DecodeBinary(ci *pgtype.ConnInfo, src []byte) error {
+	a := pgtype.Int4{}
+	b := pgtype.Text{}
+
+	fieldIter, fieldCount, err := binary.NewRecordFieldIterator(src)
+	if err != nil {
+		return err
+	}
+
+	if 2 != fieldCount {
+		return errors.Errorf("can't scan row value, number of fields don't match: found=%d expected=2", fieldCount)
+	}
+
+	_, fieldBytes, eof, err := fieldIter.Next()
+	if eof || err != nil {
+		return errors.New("Bad record")
+	}
+	if err = a.DecodeBinary(ci, fieldBytes); err != nil {
+		return err
+	}
+
+	_, fieldBytes, eof, err = fieldIter.Next()
+	if eof || err != nil {
+		return errors.New("Bad record")
+	}
+	if err = b.DecodeBinary(ci, fieldBytes); err != nil {
+		return err
+	}
+
+	dst.a = a.Int
+	if b.Status == pgtype.Present {
+		dst.b = &b.String
+	} else {
+		dst.b = nil
+	}
+
+	return nil
+}
+
 var x []byte
 
 func BenchmarkBinaryEncodingManual(b *testing.B) {
@@ -70,3 +110,52 @@ func BenchmarkBinaryEncodingRow(b *testing.B) {
 	}
 	x = buf
 }
+
+var dstRaw MyCompositeRaw
+
+func BenchmarkBinaryDecodingManual(b *testing.B) {
+	ci := pgtype.NewConnInfo()
+	buf, _ := MyType{4, ptrS("ABCDEFG")}.EncodeBinary(ci, nil)
+	dst := MyCompositeRaw{}
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		err := dst.DecodeBinary(ci, buf)
+		E(err)
+	}
+	dstRaw = dst
+}
+
+var dstMyType MyType
+
+func BenchmarkBinaryDecodingHelpers(b *testing.B) {
+	ci := pgtype.NewConnInfo()
+	buf, _ := MyType{4, ptrS("ABCDEFG")}.EncodeBinary(ci, nil)
+	dst := MyType{}
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		err := dst.DecodeBinary(ci, buf)
+		E(err)
+	}
+	dstMyType = dst
+}
+
+var gf1 int
+var gf2 *string
+
+func BenchmarkBinaryDecodingRow(b *testing.B) {
+	ci := pgtype.NewConnInfo()
+	buf, _ := MyType{4, ptrS("ABCDEFG")}.EncodeBinary(ci, nil)
+	var isNull bool
+	var f1 int
+	var f2 *string
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		err := pgtype.Row(&isNull, &f1, &f2).DecodeBinary(ci, buf)
+		E(err)
+	}
+	gf1 = f1
+	gf2 = f2
+}