Replaced Array#each with while loop for performance reasons

Example benchmark:
$ time build/bench/bin/mruby -e "Array.new(2_000_000){ |i| i }.index{ |i| i == 1_999_999 }"

Before:
real    0m0.934s
user    0m0.922s
sys     0m0.003s

After:
real    0m0.590s
user    0m0.583s
sys     0m0.007s

1 files changed, 6 insertions, 1 deletions

diff --git a/mrblib/array.rb b/mrblib/array.rb
index 5b9ee47c0..a75ed6223 100644
--- a/mrblib/array.rb
+++ b/mrblib/array.rb
@@ -46,7 +46,12 @@ class Array
   def collect!(&block)
     return to_enum :collect! unless block
 
-    self.each_index { |idx| self[idx] = block.call(self[idx]) }
+    idx = 0
+    len = size
+    while idx < len
+      self[idx] = block.call self[idx]
+      idx += 1
+    end
     self
   end