add string2uint64 and string2int64

[dportabella]

example code:

uint64_t string2uint64(const char *str, uint8_t base) {
  uint64_t  result = 0;
  for (int i = 0; str[i] != '\0'; i++)
    result = result * base + str[i] - '0';
  return result;
}