Abstract away Ripper::Lexer#scan in IRB::Color#scan

because 5b64d7ac6e made it hard to
understand #colorize_code for me and this change is needed for my next
commit.

lib/irb/color.rb

@@ -98,31 +98,16 @@ module IRB # :nodoc:
         "#{seq}#{text}#{clear}"
       end
 
-      def scan(code)
-        Ripper::Lexer.new(code).scan
-      end
-
       def colorize_code(code)
         return code unless colorable?
 
         symbol_state = SymbolState.new
         colored = +''
         length = 0
-        pos = [1, 0]
 
-        scan(code).each do |elem|
-          token = elem.event
-          str = elem.tok
-          expr = elem.state
+        scan(code) do |token, str, expr|
           in_symbol = symbol_state.scan_token(token)
-          next if ([elem.pos[0], elem.pos[1] + str.bytesize] <=> pos) <= 0
           str.each_line do |line|
-            if line.end_with?("\n")
-              pos[0] += 1
-              pos[1] = 0
-            else
-              pos[1] += line.bytesize
-            end
             line = Reline::Unicode.escape_for_print(line)
             if seq = dispatch_seq(token, expr, line, in_symbol: in_symbol)
               colored << seq.map { |s| "\e[#{s}m" }.join('')
@@ -142,6 +127,26 @@ module IRB # :nodoc:
 
       private
 
+      def scan(code)
+        pos = [1, 0]
+
+        Ripper::Lexer.new(code).scan.each do |elem|
+          str = elem.tok
+          next if ([elem.pos[0], elem.pos[1] + str.bytesize] <=> pos) <= 0
+
+          str.each_line do |line|
+            if line.end_with?("\n")
+              pos[0] += 1
+              pos[1] = 0
+            else
+              pos[1] += line.bytesize
+            end
+          end
+
+          yield(elem.event, str, elem.state)
+        end
+      end
+
       def dispatch_seq(token, expr, str, in_symbol:)
         if token == :on_parse_error or token == :compile_error
           TOKEN_SEQ_EXPRS[token][0]