XPath evaluation bench to test parsing times

This benchmark is simple enough that the overhead of evaluation is not
far greater than parsing. This makes it suitable for benchmarking the
performance increase of caching XPath ASTs.

benchmark/xpath/evaluator/simple_bench.rb

@@ -0,0 +1,10 @@
+require_relative '../../benchmark_helper'
+
+document = Oga.parse_xml('<root></root>')
+query    = 'root'
+
+Benchmark.ips do |bench|
+  bench.report(query.inspect) do
+    document.xpath(query)
+  end
+end