[ruby] make std::list include Enumerable.

Examples/test-suite/ruby/li_std_list_runme.rb

@@ -0,0 +1,8 @@
+require 'swig_assert'
+
+require 'li_std_list'
+
+include Li_std_list
+
+x = DoubleList.new([1,2,3])
+swig_assert_equal("[1.0]", "x.find_all{|e| e == 1 }", binding)

Lib/ruby/std_list.i

@@ -27,6 +27,7 @@
 #define %swig_list_methods(Type...) %swig_sequence_methods(Type)
 #define %swig_list_methods_val(Type...) %swig_sequence_methods_val(Type);
 
+%mixin std::list "Enumerable";
 
 %rename("delete")     std::list::__delete__;
 %rename("reject!")    std::list::reject_bang;