Move assignment check to RubyLex

[st0012]

Since assignment check relies on tokenization with Ripper, it feels like the responsibility of RubyLex. Irb#eval_input should simply get the result when calling each_top_level_statement on RubyLex.

[tompng]

There is a plan to add end_with_semicolon_check and number of parameter will increase. Then it will look like this.

@scanner.each_top_level_statement do |line, line_no, is_a, is_b, is_c|

end

How about this?

@scanner.each_top_level_statement do |line, line_no| # line_no and line is always needed.
  # IMO optional information of `line` should be explicitly calculated here
  is_a = @scanner.check_a?(line)
  is_c = @scanner.check_c?(line)
  is_d = @scanner.check_d?(line)
end

[st0012]

For the assignment check, I think it's better to be handled from each_top_level_statement so it's always calculated before line evaluation. I'm fine using the suggested pattern for semicolon check though.

I was also thinking instead of returning individual information as the yielding argument, perhaps we can encapsulate them inside a Statement object? But I will explore it after this and #669 is merged.