Implement fixed point type of other width

`fix16_t` is of 16.16 format, so it's 32-bit value. A500 & A600 have 16-bit data bus, so 8 and 16-bit vars are faster than 32-bit to fetch. Implementing `fix8_t` with format 8.8 analogous to `fix16_t` would speed things a bit at a cost of precision.