Add XPath //a Benchmark

## Benchmark (Comparison with rexml 3.4.1)
```
$ benchmark-driver benchmark/xpath.yaml
Calculating -------------------------------------
                     rexml 3.4.1      master  3.4.1(YJIT)  master(YJIT)
                 //a      27.805     226.061       28.364       262.896 i/s -     100.000 times in 3.596469s 0.442359s 3.525573s 0.380379s

Comparison:
                              //a
        master(YJIT):       262.9 i/s
              master:       226.1 i/s - 1.16x  slower
         3.4.1(YJIT):        28.4 i/s - 9.27x  slower
         rexml 3.4.1:        27.8 i/s - 9.45x  slower
```

- YJIT=ON : 9.27x faster
- YJIT=OFF : 8.13x faster

Author: naitoh

benchmark/xpath.yaml

@@ -0,0 +1,32 @@
+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'
+
+  DIPTH = 100
+  xml   = '<a>' * DIPTH + '<a/>' + '</a>' * DIPTH
+  doc = REXML::Document.new(xml)
+
+benchmark:
+  '//a'        : REXML::XPath.match(doc, '//a')