enum.c: make each_slice and each_cons more efficient

* enum.c (enum_each_slice, enum_each_cons): make more efficient by
  allocating less and recycling block argument arrays if possible.
  [Fixes GH-596]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45589 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-04-15

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45590 b2dd03c8-39d4-4d8f-98ff-823fe69b080e