Operators: preserve type alias

Example:

```go
// Current:
func Flatten[T any]() func(Observable[[]T]) Observable[T]

// Updated:
func Flatten[T any, Slice ~[]T]() func(Observable[Slice]) Observable[T]
```

In tests:

```go
type collection []int
s1 := collection{1, 2, 3}
s2:= collection{4, 5, 6}

ro.Pipe1(
    ro.Just(s1, s2)
    ro.Flatten(),
)
```

Also update the contributing guide + the prototypes in docs/data/