`.repeat(1, 1).repeat(1, 1)` doesn't nest arrays

```ruby
require "parslet"

class Parser < Parslet::Parser
  rule(:a) do
    str("a").as(:a).repeat(1, 1).repeat(1, 1)
  end
  root(:a)
end

p Parser.new.parse("a")
```

Actual:

```
[{:a=>"a"@0}]
```

Expected:

```
[[{:a=>"a"@0}]]
```