Polish API `unsigned char*` vs. `void*`

The discussion started in #171 

instead of:
```C
int fnc(const unsigned char *in, unsigned long inlen, unsigned char *out, unsigned long *outlen)
{
   /* some code */
}
```

use rather:
```C
int fnc(const void *inptr, unsigned long inlen, void *outptr, unsigned long *outlen)
{
   const unsigned char *in = inptr
   unsigned char *out = outptr;
   /* some code */
}
```