Issue #553

-- in.cue --
a: "foo"
b: "boo"
commands: {
	#c: {
		help: "help!"
	}
}
out: """
    a is \(a)
    b is \(b)

    c is \(commands.#c.help)
    d is \(commands.#d.help)
    """
-- out/eval/stats --
Leaks:  0
Freed:  7
Reused: 3
Allocs: 4
Retain: 0

Unifications: 7
Conjuncts:    11
Disjuncts:    7
-- out/eval --
(struct){
  a: (string){ "foo" }
  b: (string){ "boo" }
  commands: (struct){
    #c: (#struct){
      help: (string){ "help!" }
    }
  }
  out: (_|_){
    // [incomplete] out: invalid interpolation: undefined field: #d:
    //     ./in.cue:8:6
    //     ./in.cue:13:21
  }
}
-- out/compile --
--- in.cue
{
  a: "foo"
  b: "boo"
  commands: {
    #c: {
      help: "help!"
    }
  }
  out: "a is \(〈0;a〉)
  b is \(〈0;b〉)
  
  c is \(〈0;commands〉.#c.help)
  d is \(〈0;commands〉.#d.help)"
}