walk the build graph in parallel

[BrianHicks]

With https://github.com/roc-lang/rbt/pull/69, we have an actual build graph. Next step: walk it in parallel!

Dependencies:

• https://github.com/roc-lang/rbt/pull/69 for the parallel graph
• https://github.com/roc-lang/rbt/issues/72 for a data store that can be accessed in parallel

We probably don't want to do this in an async context, by the way. The async_std implementation of process is unstable, and we're going to want to avoid spawning all the build tasks at once!