Add stream support

Have `InputStream` and `OutputStream` types that allow streaming I/O on files, standard in, standard out etc.

Allow converting `File` values into corresponding streams:

```elm
File.toInputStream : File (Read p) -> InputStream
File.toOutputStream : File (Write p) -> OutputStream
```

Add `stdin` and `stdout` to `Context`:

```elm
type alias Context =
{ arguments : List String
, ...
, stdin : InputStream
, stdout : OutputStream
}
```

Have `Stream` module with various ways to read from/write to streams:

```elm
Stream.write : String -> OutputStream -> Script Stream.Error ()
Stream.read : InputStream -> Script Stream.Error String
```

`Stream.read` might need to be a bit more sophisticated to be able to cleanly detect when the end of the stream has been reached...