Support nested UDTs

[thxph]

create type type1 (
    f1 text,
    f2 int,
    );

create type type2 (
    f1 frozen<type1>,
    );

create type type3 (
    f1 frozen<type2>,
    );

---
type Typ1 struct {
	gocqlx.UDT
	F1 string
	F2 int
}
type Typ2 struct {
	gocqlx.UDT
	F1 Typ1
}
type Typ3 struct {
	gocqlx.UDT
	F1 Typ2
}

Says we have the above type definitions, currently the serialization for Typ2 and Typ3 doesn't work because MarshalUDT/UnmarshalUDT doesn't wrap the UDT field with makeUDT before calling gocql's marshaling functions.

This PR updates the MarshalUDT/UnmarshalUDT functions to check if the current field is an UDT and call makeUDT if needed.