Compatibility with IRB

Instead of accessing the struct as an array, access it via methods. There are other places inside of this file already using this API (for example https://github.com/ruby/ruby/blob/e0a5c3d2b71dfad038d7562fdd33f02ffd79232d/lib/irb/ruby-lex.rb#L829-L830).

This commit moves all struct array-ish calls to use their method calls instead. It is also ~1.23 faster accessing values via a method instead of as an array according to this microbenchmark:

```ruby
Elem = Struct.new(:pos, :event, :tok, :state, :message) do
  def initialize(pos, event, tok, state, message = nil)
    super(pos, event, tok, State.new(state), message)
  end

  # ...

  def to_a
    a = super
    a.pop unless a.empty?
    a
  end
end

class ElemClass
  attr_accessor :pos, :event, :tok, :state, :message

  def initialize(pos, event, tok, state, message = nil)
    @pos = pos
    @event = event
    @tok = tok
    @state = State.new(state)
    @message = message
  end

  def to_a
    if @message
      [@pos, @event, @tok, @state, @message]
    else
      [@pos, @event, @tok, @state]
    end
  end
end

# stub state class creation for now
class State; def initialize(val); end; end
```

```ruby
Benchmark.ips do |x|
  x.report("struct") { struct[1] }
  x.report("class ") { from_class.event }
  x.compare!
end; nil
```

```
Warming up --------------------------------------
              struct     1.624M i/100ms
              class      1.958M i/100ms
Calculating -------------------------------------
              struct     17.139M (± 2.6%) i/s -     86.077M in   5.025801s
              class      21.104M (± 3.4%) i/s -    105.709M in   5.015193s

Comparison:
              class : 21103826.3 i/s
              struct: 17139201.5 i/s - 1.23x  (± 0.00) slower
```

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