Merge/join defined before their inputs can be defined - allow inputs to forward reference names before definition about hydroflow

[MingweiSamuel]

• Current surface syntax causes a bit extra cognitive load because when pipelines merge or join, the merge/join must be defined before the inputs to that merge/join can be hooked up.

        new_exposed = (
            join() ->
            ...
        );
        (contacts -> [0]new_exposed);
        (exposed -> [1]new_exposed);

[MingweiSamuel]

allow inputs to reference names which are not yet defined

pathological cases?

should fail?

a = identity() -> b;
b = identity() -> a;

c = identity() -> c;

d = d -> identity();

a = a;

b = b -> b;

c = c -> identity() -> c;

?