Small refactorings in arrays.cpp and math.cpp

allot_array_1 and allot_array_2 are unused. And `factor_vm::unbox_array_size_slow`was written in a weird way which I think I've been able to simplify
I thought the enums were ordered in "topics." E.g ERROR_ARRAY_SIZE and ERROR_OUT_OF_FIXNUM_RANGE are both about range limits so I wanted to keep them close. :)