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')      29.130     227.729      108.669       497.976 i/s -     100.000 times in 3.432933s 0.439119s 0.920222s 0.200813s
REXML::XPath.match([REXML::Document.new(xml), REXML::Document.new(xml)], 'a//a')      13.327     225.863       43.887       496.019 i/s -     100.000 times in 7.503386s 0.442747s 2.278585s 0.201605s

Comparison:
                                         REXML::XPath.match(REXML::Document.new(xml), 'a//a')
                                                                    master(YJIT):       498.0 i/s
                                                                          master:       227.7 i/s - 2.19x  slower
                                                                     3.4.1(YJIT):       108.7 i/s - 4.58x  slower
                                                                     rexml 3.4.1:        29.1 i/s - 17.10x  slower

             REXML::XPath.match([REXML::Document.new(xml), REXML::Document.new(xml)], 'a//a')
                                                                    master(YJIT):       496.0 i/s
                                                                          master:       225.9 i/s - 2.20x  slower
                                                                     3.4.1(YJIT):        43.9 i/s - 11.30x  slower
                                                                     rexml 3.4.1:        13.3 i/s - 37.22x  slower
```

- YJIT=ON : 4.58x - 11.30x faster
- YJIT=OFF : 7.81x - 16.95 faster

Author: naitoh

benchmark/xpath.yaml

@@ -0,0 +1,34 @@
+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
+  doc1  = REXML::Document.new(xml)
+  doc2  = REXML::Document.new(xml)
+
+benchmark:
+  "REXML::XPath.match(REXML::Document.new(xml), 'a//a')" : REXML::XPath.match(doc1, "a//a")
+  "REXML::XPath.match([REXML::Document.new(xml), REXML::Document.new(xml)], 'a//a')" : REXML::XPath.match([doc1, doc2], "a//a")