Benchmark Oga XPath evaluation without the parser.

This gives better insight in the performance of the evaluator itself.

benchmark/xpath/parser/comparing_gems_bench.rb

@@ -14,6 +14,9 @@ rex_doc  = REXML::Document.new(xml)
 ox_exp    = 'number/^Text'
 xpath_exp = 'root/number/text()'
 
+oga_ast   = Oga::XPath::Parser.new(xpath_exp).parse
+evaluator = Oga::XPath::Evaluator.new(oga_doc)
+
 Benchmark.ips do |bench|
   # Technically not XPath but it's the closest thing Ox provides.
   bench.report 'Ox' do
@@ -28,6 +31,12 @@ Benchmark.ips do |bench|
     oga_doc.xpath(xpath_exp)
   end
 
+  # This is measured to see what the performance of the evaluator is _without_
+  # the overhead of the lexer/parser.
+  bench.report 'Oga cached' do
+    evaluator.evaluate_ast(oga_ast)
+  end
+
   bench.report 'REXML' do
     REXML::XPath.match(rex_doc, xpath_exp)
   end