added dots2env

The `dots2env` extracts promises from `...`. Works like this: 

```
f <- function(...) {
  res <- dots2env(environment())
}
```

It needs the environment within the function as a starting point. There is a possibility to lift this requirement, so that we could just call `dots2env()` but I'd need some "forbidden" api such as `R_GlobalContext`

The environment is constructed as a child of the environment passed in. Maybe it should instead be a child of the empty env.