Add parse_cdata benchmark

```
$ benchmark-driver benchmark/parse_cdata.yaml
Calculating -------------------------------------
                     rexml 3.2.6      master  3.2.6(YJIT)  master(YJIT)
                 dom      72.316     190.236       71.727       184.140 i/s -     100.000 times in 1.382811s 0.525662s 1.394183s 0.543065s
                 sax      74.221     202.640       73.290       199.295 i/s -     100.000 times in 1.347336s 0.493487s 1.364445s 0.501768s
                pull      74.035     202.206       73.616       199.384 i/s -     100.000 times in 1.350705s 0.494546s 1.358399s 0.501544s
              stream      74.082     201.896       73.861       197.611 i/s -     100.000 times in 1.349858s 0.495304s 1.353894s 0.506045s

Comparison:
                              dom
              master:       190.2 i/s
        master(YJIT):       184.1 i/s - 1.03x  slower
         rexml 3.2.6:        72.3 i/s - 2.63x  slower
         3.2.6(YJIT):        71.7 i/s - 2.65x  slower

                              sax
              master:       202.6 i/s
        master(YJIT):       199.3 i/s - 1.02x  slower
         rexml 3.2.6:        74.2 i/s - 2.73x  slower
         3.2.6(YJIT):        73.3 i/s - 2.76x  slower

                             pull
              master:       202.2 i/s
        master(YJIT):       199.4 i/s - 1.01x  slower
         rexml 3.2.6:        74.0 i/s - 2.73x  slower
         3.2.6(YJIT):        73.6 i/s - 2.75x  slower

                           stream
              master:       201.9 i/s
        master(YJIT):       197.6 i/s - 1.02x  slower
         rexml 3.2.6:        74.1 i/s - 2.73x  slower
         3.2.6(YJIT):        73.9 i/s - 2.73x  slower
```

Author: naitoh

benchmark/parse_cdata.yaml

@@ -0,0 +1,50 @@
+loop_count: 100
+contexts:
+  - gems:
+      rexml: 3.2.6
+    require: false
+    prelude: require 'rexml'
+  - name: master
+    prelude: |
+      $LOAD_PATH.unshift(File.expand_path("lib"))
+      require 'rexml'
+  - name: 3.2.6(YJIT)
+    gems:
+      rexml: 3.2.6
+    require: false
+    prelude: |
+      require 'rexml'
+      RubyVM::YJIT.enable
+  - name: master(YJIT)
+    prelude: |
+      $LOAD_PATH.unshift(File.expand_path("lib"))
+      require 'rexml'
+      RubyVM::YJIT.enable
+
+prelude: |
+  require 'rexml/document'
+  require 'rexml/parsers/sax2parser'
+  require 'rexml/parsers/pullparser'
+  require 'rexml/parsers/streamparser'
+  require 'rexml/streamlistener'
+
+  def build_xml(n_depth)
+    xml = "<?xml version=\"1.0\"?>\n" +
+           "<root>Test</root>\n" +
+           "<!" + "[CDATA[" * n_depth + "]]>\n"
+  end
+  xml = build_xml(100000)
+
+  class Listener
+    include REXML::StreamListener
+  end
+
+benchmark:
+  'dom'        : REXML::Document.new(xml)
+  'sax'        : REXML::Parsers::SAX2Parser.new(xml).parse
+  'pull'       : |
+    parser = REXML::Parsers::PullParser.new(xml)
+    while parser.has_next?
+      parser.pull
+    end
+  'stream'     : REXML::Parsers::StreamParser.new(xml, Listener.new).parse