[wip] Reuse instance when generate_line

I'm still thinking...

I used benchmark script as below:
fluent/fluentd#2529

Warming up --------------------------------------
                 now     5.553k i/100ms
                 new    10.626k i/100ms
            instance     9.009k i/100ms
Calculating -------------------------------------
                 now     57.255k (± 4.1%) i/s -    288.756k in   5.051981s
                 new    114.090k (± 7.1%) i/s -    573.804k in   5.062333s
            instance     95.062k (± 4.1%) i/s -    477.477k in   5.031413s

lib/csv.rb

@@ -414,6 +414,7 @@ def initialize(message, line_number)
     skip_lines:         nil,
     liberal_parsing:    false,
     quote_empty:        true,
+    use_last_instance:  false,
   }.freeze
 
   #
@@ -565,7 +566,19 @@ def self.generate_line(row, **options)
     elsif field = row.find {|f| f.is_a?(String)}
       str.force_encoding(field.encoding)
     end
-    (new(str, options) << row).string
+    if options[:use_last_instance]
+      csv = last_instance(str, options)
+      line = (csv << row).string.dup
+      csv.rewind
+      csv.truncate(0)
+      line
+    else
+      (new(str, options) << row).string
+    end
+  end
+
+  def self.last_instance(data, **options)
+    (@@instance ||= {})[Thread.current] ||= new(data, options)
   end
 
   #
@@ -936,7 +949,8 @@ def initialize(data,
                  write_converters: nil,
                  write_nil_value: nil,
                  write_empty_value: "",
-                 strip: false)
+                 strip: false,
+                 use_last_instance: false)
     raise ArgumentError.new("Cannot parse nil as CSV") if data.nil?
 
     # create the IO object we will read from