allow redirecting file descriptors for the current shell

There's a feature of bash that allows one to set redirections for the current shell. It's used by running [`exec`](http://wiki.bash-hackers.org/commands/builtin/exec) without a command.

> If only redirections are given, the redirections affect the current shell without executing any program.

I've searched and as far as I can tell this is not possible with fish.