Allow `generator` to take any function

Mostly because it's useful to do this:

```R
openai_chat_stream <- function(...) {...}

.onLoad <- function(...) {
  openai_chat_stream <<- coro::generator(openai_chat_stream)
}
```

Commit that made this an error: https://github.com/r-lib/coro/commit/c3791eb41e3a1ac5f951daef9f8a11ac02ed8f30