Reorder Sinatra::Base's metaclass

`Sinatra::Base`'s metaclass (`class << self`) has a lot of `public` and `private` methods. For instance:

```ruby
class Base
  # methods
  class << self
    # methods

    private

    # methods

    public
    
    #methods

    private

    # methods

    public

    # methods
  end
end
```

and so on.

So this PR is merely organizational, it organizes the code moving *all* public methods to the public section and *all* private methods to the private section so we have only one public section and one private section. Other change is to move constants and `attr_accessor`s to the top of the class.