Add String#concat

1. I don't know about how fast `String#concat` vs `String#+=`, but i think first is more semantic
2. I don't know how i can add `Numeric#to_str` ... if i do something like this, it make specs red in `Array`

```ruby
  def to_s(base = 10)
    if base < 2 || base > 36
      raise ArgumentError, 'base must be between 2 and 36'
    end

    `self.toString(base)`
  end

  alias inspect to_s
  alias to_str to_s
```

This article http://www.sitepoint.com/javascript-fast-string-concatenation/ says that challenge in `String#concat` vs `Array.join` mode, not in `String#concat` vs `String#+=`