Use sizeof(void*) instead of mrb_int size and extend embed string size.

Since `mrb_int` is `uint32_t` by default independently of pointer size, embed string size should use pointer size. Object in RVALUE's max use is 3 pointers so I used `sizeof(void*) * 3`.
In additional doing compile time thing in C is troublesome, extending embed string size to 5bit will make `RSTRING_EMBED_LEN_MAX` more easier.