feat(hydroflow): implement flo loop syntax parsing

[MingweiSamuel]

Just the parsing. Error checking not yet implemented, semantics/scheduling not yet implemented.

users = source_stream(0..);
messages = source_stream(0..);
loop {
    users -> batch() -> [0]cp;
    messages -> batch() -> [1]cp;
    cp = cross_join() -> for_each(|(user, message)| println!("notify {} of {}", user, message));
}

(batch() does not yet exist)