next thing to do is figure out how LazyPath is supposed to work now.
something like this:
* each Step that provides LazyPath objects has a setLazyPath and
getLazyPath function which takes a tagged union identifying which one
to access
* steps that fulfill LazyPath objects can freely call setLazyPath
without obtaining a lock because the dependency graph prevents
simultaneous access.
* similarly, steps that access LazyPath results can freely call
getLazyPath without obtaining a lock, because after modification,
there may be simultaneous reads from dependencies but they will all be
read-only
* a fulfilled LazyPath object is a read-only std.Build.Cache.Path.
Andrew Kelley