Implement cons

lib/chervil/core.rb

         else
           error
         end
+      end,
+      "cons" => Proc.new do |args|
+        error = arity_check(args, 2)
+        if error.nil?
+          el, els = args
+          error = type_check([els], Array)
+          if error.nil?
+            els.prepend(el)
+          end
+        end
       end
     }
   end

spec/core_spec.rb

         Error.new("`car` expects a non-empty list")
       )
     end
+
+    it 'implements cons' do
+      expect(Core::CORE['cons'].call([1.0, [2.0, 3.0]])).to eq(
+        [1.0, 2.0, 3.0]
+      )
+    end
   end
 end