Add dumb loop microbenchmark (#349)

This is a modified version of a microbenchmark circulated on
twitter by "TheDumbTechGuy". It's obviously not representative of
most Ruby code, but it does show areas where we could perform
much better with better inlining and better quality code
generation.

benchmarks.yml

@@ -147,3 +147,7 @@ ruby-xor:
   desc: pure-Ruby string XOR microbenchmark, analogous to xorcist C extension.
   category: micro
   single_file: true
+loops-times:
+  desc: nested loop Integer#times and array access microbenchmark
+  category: micro
+  single_file: true

benchmarks/loops-times.rb

@@ -0,0 +1,21 @@
+require_relative '../harness/loader'
+
+# Fix these values for determinism
+u = 5
+r = 7
+
+run_benchmark(10) do
+  a = Array.new(10000, 0)
+
+  4_000.times do |i|
+    4_000.times do |j|
+      a[i] += j % u
+    end
+    a[i] += r
+  end
+
+  result = a[r]
+  if result != 8007
+    raise "incorrect result"
+  end
+end