Const completion & document using Module.nesting

[tompng]

class AA
  class AB1
    AC1 = 1
  end
  class AB2
    AC2 = 2
    AB1.instance_eval { binding.irb }
  end
end

irb> A█

before: AA, ARGF, ARGV, ArgumentError, Array
after: AA, AB1, AB2, AC2, ARGF, ARGV, ArgumentError, Array

Not done in this pull request

irb> class ::AA::AB1
irb>   A█
should complete AC1

irb> class ::AA::AB1
irb>   @@a█
should complete class variables of ::AA::AB1

[tompng]

Planning to try this again after #28