datetime: do not mess with nsec in interval

Do not even try to make more readable output of secs/nsec,
but rather report them as is, without any [de]normalization.

Not the prior way:
```
tarantool> dt.interval.new{min=1, sec=59, nsec=2e9+1}
--
- +1 minutes, 61.000000001 seconds
...
```

But instead as:
```
tarantool> dt.interval.new{min=1, sec=59, nsec=2e9+1}
--
- +1 minutes, 59 seconds, 2000000001 nanoseconds
...
```

Closes #7045

NO_DOC=internal