modular import of hydroflow graphs

[jhellerstein]

It would be good to allow individual hydroflow graph objects to be combined into a bigger graph. Ideally we should be able to take a subgraph S and instantiate it multiple times with different names.

This likely require syntax for explicit "inputs" and "outputs" for these graphs.

Here's how it was done in Bloom.

[MingweiSamuel]

Probably too big to try to get in 0.2

[jhellerstein]

V1 is local-only Hydroflow (source text import)
then we'll look at Rust boilerplate for networking

[zzlk]

fixed by: #898