Add benchmark

## Benchmark
```
$ benchmark-driver benchmark/parse_doctype.yaml
Calculating -------------------------------------
                             before       after  before(YJIT)  after(YJIT)
      single_entity(dom)    22.381k     24.492k        5.098k       5.211k i/s -     100.000 times in 0.004468s 0.004083s 0.019615s 0.019189s
   single_entity(stream)    46.882k     48.054k        8.796k       8.691k i/s -     100.000 times in 0.002133s 0.002081s 0.011369s 0.011506s
   chained_entities(dom)     2.665k      2.531k        2.349k       2.340k i/s -     100.000 times in 0.037520s 0.039509s 0.042574s 0.042726s
chained_entities(stream)     2.455k      2.387k        2.351k       2.290k i/s -     100.000 times in 0.040732s 0.041888s 0.042538s 0.043660s
      many_entities(dom)    675.927     679.094       684.308      674.686 i/s -     100.000 times in 0.147945s 0.147255s 0.146133s 0.148217s
   many_entities(stream)     2.602k      2.501k        2.403k       2.276k i/s -     100.000 times in 0.038427s 0.039986s 0.041607s 0.043944s
    repeated_entity(dom)    419.090     414.295       445.286      444.575 i/s -     100.000 times in 0.238612s 0.241374s 0.224575s 0.224934s
 repeated_entity(stream)     1.449k      1.401k        1.590k       1.582k i/s -     100.000 times in 0.069014s 0.071358s 0.062898s 0.063201s

Comparison:
                   single_entity(dom)
                   after:     24491.8 i/s
                  before:     22381.4 i/s - 1.09x  slower
             after(YJIT):      5211.3 i/s - 4.70x  slower
            before(YJIT):      5098.1 i/s - 4.80x  slower

                single_entity(stream)
                   after:     48053.8 i/s
                  before:     46882.3 i/s - 1.02x  slower
            before(YJIT):      8795.8 i/s - 5.46x  slower
             after(YJIT):      8691.1 i/s - 5.53x  slower

                chained_entities(dom)
                  before:      2665.2 i/s
                   after:      2531.1 i/s - 1.05x  slower
            before(YJIT):      2348.9 i/s - 1.13x  slower
             after(YJIT):      2340.5 i/s - 1.14x  slower

             chained_entities(stream)
                  before:      2455.1 i/s
                   after:      2387.3 i/s - 1.03x  slower
            before(YJIT):      2350.8 i/s - 1.04x  slower
             after(YJIT):      2290.4 i/s - 1.07x  slower

                   many_entities(dom)
            before(YJIT):       684.3 i/s
                   after:       679.1 i/s - 1.01x  slower
                  before:       675.9 i/s - 1.01x  slower
             after(YJIT):       674.7 i/s - 1.01x  slower

                many_entities(stream)
                  before:      2602.3 i/s
                   after:      2500.9 i/s - 1.04x  slower
            before(YJIT):      2403.4 i/s - 1.08x  slower
             after(YJIT):      2275.6 i/s - 1.14x  slower

                 repeated_entity(dom)
            before(YJIT):       445.3 i/s
             after(YJIT):       444.6 i/s - 1.00x  slower
                  before:       419.1 i/s - 1.06x  slower
                   after:       414.3 i/s - 1.07x  slower

              repeated_entity(stream)
            before(YJIT):      1589.9 i/s
             after(YJIT):      1582.3 i/s - 1.00x  slower
                  before:      1449.0 i/s - 1.10x  slower
                   after:      1401.4 i/s - 1.13x  slower
```
- YJIT=ON : 0.95 - 1.09x faster
- YJIT=OFF : 0.94 - 1.02x faster

Author: naitoh

benchmark/parse_doctype.yaml

@@ -0,0 +1,77 @@
+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/streamparser'
+  require 'rexml/streamlistener'
+
+  # Single entity reference: <!ENTITY foo "bar">  ->  &foo;
+  single_entity_xml = <<~XML
+    <!DOCTYPE root [
+      <!ENTITY word "hello">
+    ]>
+    <root>&word;</root>
+  XML
+
+  # Chained entity references: a -> b -> c -> ... -> value (n_depth levels deep)
+  n_depth = 100
+  chained_entities_xml = <<~XML
+    <!DOCTYPE root [
+      #{(1..n_depth).map {|i| i == n_depth ? "<!ENTITY e#{i} \"value\">" : "<!ENTITY e#{i} \"&e#{i+1};\">" }.join("\n")}
+    ]>
+    <root>&e1;</root>
+  XML
+
+  # Many distinct entities referenced in document content
+  n_entities = 100
+  many_entities_xml = <<~XML
+    <!DOCTYPE root [
+      #{(1..n_entities).map {|i| "<!ENTITY ent#{i} \"value#{i}\">" }.join("\n")}
+    ]>
+    <root>#{(1..n_entities).map {|i| "&ent#{i};" }.join}</root>
+  XML
+
+  # Same entity referenced repeatedly in attribute values
+  n_items = 100
+  repeated_entity_xml = <<~XML
+    <!DOCTYPE root [
+      <!ENTITY word "hello">
+    ]>
+    <root>#{'<item val="&word;">&word;</item>' * n_items}</root>
+  XML
+
+  class Listener
+    include REXML::StreamListener
+  end
+
+benchmark:
+  'single_entity(dom)'      : REXML::Document.new(single_entity_xml).root.text
+  'single_entity(stream)'   : REXML::Parsers::StreamParser.new(single_entity_xml, Listener.new).parse
+  'chained_entities(dom)'   : REXML::Document.new(chained_entities_xml).root.text
+  'chained_entities(stream)': REXML::Parsers::StreamParser.new(chained_entities_xml, Listener.new).parse
+  'many_entities(dom)'      : REXML::Document.new(many_entities_xml).root.text
+  'many_entities(stream)'   : REXML::Parsers::StreamParser.new(many_entities_xml, Listener.new).parse
+  'repeated_entity(dom)'    : REXML::Document.new(repeated_entity_xml).root.text
+  'repeated_entity(stream)' : REXML::Parsers::StreamParser.new(repeated_entity_xml, Listener.new).parse