walk the build graph in parallel

[BrianHicks]

With #69, we have an actual build graph. Next step: walk it in parallel!

Dependencies:

• allow jobs to depend on jobs #69 for the parallel graph
• move to a data store that can be used across threads #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!