Integrate kazuho/mruby-class-new-fiber-safe in the master.

Avoid calling `initialize` via `mrb_funcall`, which cause `cross C
boundary` error from Fibers started in the method.

mrblib/00class.rb

@@ -1,3 +1,11 @@
+class Class
+  def new(*args, &block)
+    obj = self.allocate
+    obj.initialize(*args, &block)
+    obj
+  end
+end
+
 class Module
    # 15.2.2.4.12
   def attr_accessor(*names)

src/class.c

@@ -2159,6 +2159,7 @@ mrb_init_class(mrb_state *mrb)
   mrb_define_method(mrb, bob, "instance_eval",           mrb_obj_instance_eval,    MRB_ARGS_ANY());  /* 15.3.1.3.18 */
 
   mrb_define_class_method(mrb, cls, "new",               mrb_class_new_class,      MRB_ARGS_OPT(1));
+  mrb_define_method(mrb, cls, "allocate",                mrb_instance_alloc,       MRB_ARGS_NONE());
   mrb_define_method(mrb, cls, "superclass",              mrb_class_superclass,     MRB_ARGS_NONE()); /* 15.2.3.3.4 */
   mrb_define_method(mrb, cls, "new",                     mrb_instance_new,         MRB_ARGS_ANY());  /* 15.2.3.3.3 */
   mrb_define_method(mrb, cls, "initialize",              mrb_class_initialize,     MRB_ARGS_OPT(1)); /* 15.2.3.3.1 */