Remove Struct in favor of Class

One Struct remains in `table.rb` `Table::Rows`.
The method `[]` is used: 
```ruby
NoMethodError: undefined method `[]' for #<Asciidoctor::Table::Rows:0x00000005d81030>
    (erb):19:in `block in get_binding'
    (erb):19:in `select'
    (erb):19:in `get_binding'
```