Boxing:

* `MRB_NO_BOXING`   (`mrb_value` packed in `struct`)
* `MRB_WORD_BOXING` (`mrb_value` packed in `struct`) default
* `MRB_NAN_BOXING`  (`mrb_value` packed in `double`)

`mrb_int` size

* `MRB_INT32`       (`int32_t` as `mrb_int`)
* `MRB_INT64`       (`int64` as `mrb_int`) conflict with `MRB_NAN_BOXING'

Architecture

* `MRB_64BIT`      (`sizeof(void*)` is 64 bits)
* `MRB_32BIT`      (`sizeof(void*)` is 32 bits)