Automated UDT support

This patch adds the power of GocqlX to UDTs.
Now you can make a struct be UDT compatible by adding a single line.

```
type FullName struct {
	gocqlx.UDT
	FirstName string
	LastName  string
}
```

Signed-off-by: Michał Matczuk <michal@scylladb.com>

iterx_test.go

@@ -36,18 +36,13 @@ func (n *FullName) UnmarshalCQL(info gocql.TypeInfo, data []byte) error {
 }
 
 type FullNameUDT struct {
-	FirstName string
-	LastName  string
+	gocqlx.UDT
+	FullName
 }
 
-func (n FullNameUDT) MarshalUDT(name string, info gocql.TypeInfo) ([]byte, error) {
-	f := gocqlx.DefaultMapper.FieldByName(reflect.ValueOf(n), name)
-	return gocql.Marshal(info, f.Interface())
-}
-
-func (n *FullNameUDT) UnmarshalUDT(name string, info gocql.TypeInfo, data []byte) error {
-	f := gocqlx.DefaultMapper.FieldByName(reflect.ValueOf(n), name)
-	return gocql.Unmarshal(info, data, f.Addr().Interface())
+type FullNamePtrUDT struct {
+	gocqlx.UDT
+	*FullName
 }
 
 func TestStruct(t *testing.T) {
@@ -75,7 +70,8 @@ func TestStruct(t *testing.T) {
 			testvarint     varint,
 			testinet       inet,
 			testcustom     text,
-			testudt        gocqlx_test.FullName
+			testudt        gocqlx_test.FullName,
+			testptrudt     gocqlx_test.FullName
 		)`); err != nil {
 		t.Fatal("create table:", err)
 	}
@@ -98,6 +94,7 @@ func TestStruct(t *testing.T) {
 		Testinet      string
 		Testcustom    FullName
 		Testudt       FullNameUDT
+		Testptrudt    FullNamePtrUDT
 	}
 
 	bigInt := new(big.Int)
@@ -122,10 +119,11 @@ func TestStruct(t *testing.T) {
 		Testvarint:    bigInt,
 		Testinet:      "213.212.2.19",
 		Testcustom:    FullName{FirstName: "John", LastName: "Doe"},
-		Testudt:       FullNameUDT{FirstName: "John", LastName: "Doe"},
+		Testudt:       FullNameUDT{FullName: FullName{FirstName: "John", LastName: "Doe"}},
+		Testptrudt:    FullNamePtrUDT{FullName: &FullName{FirstName: "John", LastName: "Doe"}},
 	}
 
-	if err := session.Query(`INSERT INTO struct_table (testuuid, testtimestamp, testvarchar, testbigint, testblob, testbool, testfloat,testdouble, testint, testdecimal, testlist, testset, testmap, testvarint, testinet, testcustom, testudt) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+	if err := gocqlx.Query(session.Query(`INSERT INTO struct_table (testuuid, testtimestamp, testvarchar, testbigint, testblob, testbool, testfloat,testdouble, testint, testdecimal, testlist, testset, testmap, testvarint, testinet, testcustom, testudt, testptrudt) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`), nil).Bind(
 		m.Testuuid,
 		m.Testtimestamp,
 		m.Testvarchar,
@@ -142,7 +140,8 @@ func TestStruct(t *testing.T) {
 		m.Testvarint,
 		m.Testinet,
 		m.Testcustom,
-		m.Testudt).Exec(); err != nil {
+		m.Testudt,
+		m.Testptrudt).Exec(); err != nil {
 		t.Fatal("insert:", err)
 	}
 
@@ -186,6 +185,28 @@ func TestStruct(t *testing.T) {
 			t.Fatal("not equals")
 		}
 	})
+
+	t.Run("struct scan", func(t *testing.T) {
+		var (
+			v StructTable
+			n int
+		)
+
+		i := gocqlx.Query(session.Query(`SELECT * FROM struct_table`), nil).Iter()
+		for i.StructScan(&v) {
+			n++
+		}
+		if err := i.Close(); err != nil {
+			t.Fatal("struct scan failed", err)
+		}
+
+		if n != 1 {
+			t.Fatal("struct scan unexpected number of rows", n)
+		}
+		if !reflect.DeepEqual(m, v) {
+			t.Fatal("not equals")
+		}
+	})
 }
 
 func TestScannable(t *testing.T) {
@@ -320,7 +341,12 @@ func TestStructOnlyUDT(t *testing.T) {
 		t.Fatal("create table:", err)
 	}
 
-	m := FullNameUDT{"John", "Doe"}
+	m := FullNameUDT{
+		FullName: FullName{
+			FirstName: "John",
+			LastName:  "Doe",
+		},
+	}
 
 	if err := session.Query(`INSERT INTO struct_only_udt_table (first_name, last_name) values (?, ?)`, m.FirstName, m.LastName).Exec(); err != nil {
 		t.Fatal("insert:", err)
@@ -401,7 +427,7 @@ func TestUnsafe(t *testing.T) {
 	t.Run("safe get", func(t *testing.T) {
 		var v UnsafeTable
 		i := gocqlx.Iter(session.Query(`SELECT * FROM unsafe_table`))
-		if err := i.Get(&v); err == nil || err.Error() != "missing destination name \"testtextunbound\" in *gocqlx_test.UnsafeTable" {
+		if err := i.Get(&v); err == nil || err.Error() != "missing destination name \"testtextunbound\" in gocqlx_test.UnsafeTable" {
 			t.Fatal("expected ErrNotFound", "got", err)
 		}
 	})
@@ -409,7 +435,7 @@ func TestUnsafe(t *testing.T) {
 	t.Run("safe select", func(t *testing.T) {
 		var v []UnsafeTable
 		i := gocqlx.Iter(session.Query(`SELECT * FROM unsafe_table`))
-		if err := i.Select(&v); err == nil || err.Error() != "missing destination name \"testtextunbound\" in *gocqlx_test.UnsafeTable" {
+		if err := i.Select(&v); err == nil || err.Error() != "missing destination name \"testtextunbound\" in gocqlx_test.UnsafeTable" {
 			t.Fatal("expected ErrNotFound", "got", err)
 		}
 		if cap(v) > 0 {
@@ -510,6 +536,40 @@ func TestNotFound(t *testing.T) {
 	})
 }
 
+func TestErrorOnNil(t *testing.T) {
+	session := CreateSession(t)
+	defer session.Close()
+	if err := ExecStmt(session, `CREATE TABLE gocqlx_test.nil_table (testtext text PRIMARY KEY)`); err != nil {
+		t.Fatal("create table:", err)
+	}
+
+	const (
+		stmt   = "SELECT * FROM not_found_table WRONG"
+		golden = "expected a pointer but got <nil>"
+	)
+
+	t.Run("get", func(t *testing.T) {
+		err := gocqlx.Iter(session.Query(stmt)).Get(nil)
+		if err == nil || err.Error() != golden {
+			t.Fatalf("Get()=%q expected %q error", err, golden)
+		}
+	})
+	t.Run("select", func(t *testing.T) {
+		err := gocqlx.Iter(session.Query(stmt)).Select(nil)
+		if err == nil || err.Error() != golden {
+			t.Fatalf("Select()=%q expected %q error", err, golden)
+		}
+	})
+	t.Run("struct scan", func(t *testing.T) {
+		i := gocqlx.Iter(session.Query(stmt))
+		i.StructScan(nil)
+		err := i.Close()
+		if err == nil || err.Error() != golden {
+			t.Fatalf("StructScan()=%q expected %q error", err, golden)
+		}
+	})
+}
+
 func TestPaging(t *testing.T) {
 	session := CreateSession(t)
 	defer session.Close()