Add benchmark for Array#combination

benchmark/core/array/bench_combination.rb

@@ -0,0 +1,24 @@
+require 'benchmark'
+require 'benchmark/ips'
+require File.expand_path('../shared_array.rb', __FILE__)
+
+Benchmark.ips do |x|
+  small_array = $small_array.dup
+
+  x.report "combination by groups of 2" do |times|
+    i = 0
+    while i < times
+      small_array.combination(2).to_a
+      i += 1
+    end
+  end
+
+  x.report "combination by groups of 4" do |times|
+    i = 0
+    while i < times
+      small_array.combination(4).to_a
+      i += 1
+    end
+  end
+
+end