Add mealplan resolution to DslLoader

[temujin9]

(This depends on #278, #279, and #280. Do them first.)

Add an abstraction to Ironfan that allows users to say env :aha; mealplan :foo_bar { baz 10; fwee 11 } in their cluster file, and treat the result of [scope]::Foo::Bar.resolve(baz = 1, fwee = 1) (as loaded from environments/aha/cookbooks/foo/mealplan/bar.rb) as if it were written into the cluster file itself.