fixes #309. Fixes #310

[o-smirnov]

No description provided.

[JSKenyon]

This looks uncontroversial to me, provided that removing the flatten doesn't have unforeseen consequences.

[o-smirnov]

It does have consequences for hierarchical assignments i.e.

assign:
  log: 
    dir: foo

might not work as expected anymore (has to be assign: log.dir: foo)

I don't think we have recipes in the wild using the former.

[o-smirnov]

P.S. I do feel the assign logic overall is still a little unsettled in places, but I'm happy to muddle forward and let us refine it with more experience.