Add benchmark for Config#to_h (#152)

This gives a good indication of overall config read performance

benchmarks/config_to_h_benchmark.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+require "bundler/setup"
+
+require "benchmark/ips"
+require "dry/configurable"
+
+class Base
+  extend Dry::Configurable
+
+  setting :foo
+  setting :bar
+  setting :baz, default: "baz"
+  setting :qux, default: "qux", constructor: :upcase.to_proc
+
+  config.bar = "bar"
+end
+
+class Sub1 < Base
+  setting :quux
+end
+
+class Sub2 < Sub1
+end
+
+def name(str)
+  [ENV["BENCHMARK_NAME"], str].compact.join("/")
+end
+
+Benchmark.ips do |x|
+  x.warmup = 1
+  x.time = 3
+
+  x.report(name("base")) do
+    Base.config.to_h
+  end
+
+  x.report(name("sub")) do
+    Sub2.config.to_h
+  end
+
+  x.save! ENV["SAVE_FILE"] if ENV["SAVE_FILE"]
+  x.compare!
+end