[Feature] syntactic sugar for piping streams

Presumably it is currently possible to pipe inside a `$` command?

```
echo $ ls | wc $
```

But it would be useful to establish some kind of native syntax for piping that would work even if the commands were wrapped in functions. Something like this:

```
pub fun list(path): Pipe {
yield from $ ls -l {path} $
...do other stuff...
}
pub fun count_words(input: Pipe) {
feed input to $ wc $
}

list(".") => pipe1
pipe1 => count_words
list(".") => count_words
```

That general idea anyway. It'd need to support asynchronous commands, both self-forking and non-self-forking. Or possibly some kind of "composition":

```
cmd = compose list(".") => count_words
run cmd
```