Add benchmark for various composite encoder implementations

```
BenchmarkBinaryEncodingManual-12   824053234   28.9 ns/op    0 B/op     0 allocs/op
BenchmarkBinaryEncodingHelper-12   76815436    314 ns/op     192 B/op   5 allocs/op
BenchmarkBinaryEncodingRow-12      65302958    364 ns/op     192 B/op   5 allocs/op
```

composite_bench_test.go

@@ -0,0 +1,70 @@
+package pgtype_test
+
+import (
+	"testing"
+
+	"github.com/jackc/pgtype"
+	"github.com/jackc/pgtype/binary"
+)
+
+type MyCompositeRaw struct {
+	a int32
+	b *string
+}
+
+func (src MyCompositeRaw) EncodeBinary(ci *pgtype.ConnInfo, buf []byte) (newBuf []byte, err error) {
+	a := pgtype.Int4{src.a, pgtype.Present}
+
+	fieldBytes := make([]byte, 0, 64)
+	fieldBytes, _ = a.EncodeBinary(ci, fieldBytes[:0])
+
+	newBuf = binary.RecordStart(buf, 2)
+	newBuf = binary.RecordAdd(newBuf, pgtype.Int4OID, fieldBytes)
+
+	if src.b != nil {
+		fieldBytes, _ = pgtype.Text{*src.b, pgtype.Present}.EncodeBinary(ci, fieldBytes[:0])
+		newBuf = binary.RecordAdd(newBuf, pgtype.TextOID, fieldBytes)
+	} else {
+		newBuf = binary.RecordAddNull(newBuf, pgtype.TextOID)
+	}
+	return
+}
+
+var x []byte
+
+func BenchmarkBinaryEncodingManual(b *testing.B) {
+	buf := make([]byte, 0, 128)
+	ci := pgtype.NewConnInfo()
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		v := MyCompositeRaw{4, ptrS("ABCDEFG")}
+		buf, _ = v.EncodeBinary(ci, buf[:0])
+	}
+	x = buf
+}
+
+func BenchmarkBinaryEncodingHelper(b *testing.B) {
+	buf := make([]byte, 0, 128)
+	ci := pgtype.NewConnInfo()
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		v := MyType{4, ptrS("ABCDEFG")}
+		buf, _ = v.EncodeBinary(ci, buf[:0])
+	}
+	x = buf
+}
+
+func BenchmarkBinaryEncodingRow(b *testing.B) {
+	buf := make([]byte, 0, 128)
+	ci := pgtype.NewConnInfo()
+
+	b.ResetTimer()
+	for n := 0; n < b.N; n++ {
+		c := pgtype.Composite(&pgtype.Int4{}, &pgtype.Text{})
+		c.Set(pgtype.Row(2, "bar"))
+		buf, _ = c.EncodeBinary(ci, buf[:0])
+	}
+	x = buf
+}