Add single-benchmark execute, simplify compilation

performance/benchmarks/simple_loop.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+Benchmarks.define('simple_loop', Class.new do
+  TEMPLATE = "{% for i in (1..1000) %}{{ i }}{% endfor %}"
+
+  def compile
+    @parsed = Liquid::Template.parse(TEMPLATE)
+  end
+
+  def render
+    @parsed.render
+  end
+end.new)