Auto UDT support slices of UDTs

[mmatczuk]

Slices of UDTs are not supported by gocqlx. This is because gocqlx hacks binding of objects and instead passing original pointer to the object representing column, it passes special type which internally rewrites pointers. This works only for objects, because we can allocate and initialize this special type for each column. But with slices, allocation and initialization is done by the driver so gocqlx cannot interfere with it.