Add DerefWithDefault to ref package

[sevein]

This PR adds func DerefDefault[T any](x *T, d T) T which is just like ref.DerefZero but it returns the default value specified by d, instead of the zero value, when the pointer x is nil, e.g.:

x := (*string)(nil)
fmt.Println(ref.DerefDefault(x, "default")) // "default"

y := "data"
fmt.Println(ref.DerefDefault(&y, "default")) // "data"