Implement Kernel.fail and Kernel#fail .

src/kernel.c

@@ -1189,6 +1189,8 @@ mrb_init_kernel(mrb_state *mrb)
   mrb_define_method(mrb, krn, "to_s",                       mrb_any_to_s,                    MRB_ARGS_NONE());    /* 15.3.1.3.46 */
   mrb_define_method(mrb, krn, "__case_eqq",                 mrb_obj_ceqq,                    MRB_ARGS_REQ(1));    /* internal */
 
+  mrb_define_module_function(mrb, krn, "fail", mrb_f_raise, MRB_ARGS_NONE());
+
   mrb_include_module(mrb, mrb->object_class, mrb->kernel_module);
   mrb_alias_method(mrb, mrb->module_class, mrb_intern_lit(mrb, "dup"), mrb_intern_lit(mrb, "clone"));
 }

test/t/kernel.rb

@@ -563,3 +563,7 @@ assert('stack extend') do
   end
 end
 
+assert('Kernel.fail, Kernel#fail') do
+  assert_raise(RuntimeError) { fail }
+  assert_raise(RuntimeError) { Kernel.fail }
+end