Remove mrb_str_new2().

This patch will modify API set.

mrb_str_new2() is slightly rough as it throws an exception.
And it is similar to mrb_str_new_str().
And ... "new2" is not a good name... ,right? ;-)

So I suggest to use mrb_str_new_cstr() instead.
I applied some modification to mrb_str_new_cstr(). If it is given NULL pointer, it generates mrb_str that have zero length string.