Add XPath a//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)
REXML::XPath.match(REXML::Document.new(xml), 'a//a')      29.215     234.909      108.945       492.410 i/s -     100.000 times in 3.422925s 0.425697s 0.917898s 0.203083s

Comparison:
             REXML::XPath.match(REXML::Document.new(xml), 'a//a')
                                        master(YJIT):       492.4 i/s
                                              master:       234.9 i/s - 2.10x  slower
                                         3.4.1(YJIT):       108.9 i/s - 4.52x  slower
                                         rexml 3.4.1:        29.2 i/s - 16.85x  slower
```

- YJIT=ON : 4.52x faster
- YJIT=OFF : 8.04x 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'
+
+  DEPTH = 100
+  xml   = '<a>' * DEPTH + '</a>' * DEPTH
+  doc   = REXML::Document.new(xml)
+
+benchmark:
+  "REXML::XPath.match(REXML::Document.new(xml), 'a//a')" : REXML::XPath.match(doc, "a//a")