gcc 9 started complaining about type mismatches:
1. all int sized return types are expected to be
"unsigned int" instead of e.g., "uint32_t"
2. all arguments are expected to be passed as (volatile)
void * pointers
This commit fixes 1. by doing ifdef'ed typedefs for I2 and I4, an fixes
2. by using void pointers, then casting within the template bodies.