Motivation:

There is a possible overflow when computing a `Double` value for `fractionalSeconds` from an `ArraySlice` of bytes. Hence, access to this original `ArraySlice` is necessary.

Modifications:

- Add `rawFractionalSeconds` property to `GeneralizedTime`.
- Add a new `internal init` to `GeneralizedTime` that accepts `rawFractionalSeconds` instead of `fractionalSeconds` and generates the later from the former.
- Adjust the algorithm for computing `fractionalSeconds`, from a mathematical to a `String` computation. This is due to the precision mismatch between the mathematical computation and the native `Double` computation. This issue, which previously did not surface, now does, due to the newly included round-trip conversion from `fractionalSeconds` to `rawFractionalSeconds`.

Result:

- The `rawFractionalSeconds` property now provides the original `ArraySlice` from which `fractionalSeconds` was computed.
- `GeneralizedTime` can now compute `fractionalSeconds` from the provided `rawFractionalSeconds`.

- Improve error and documentation.
- Change the new `GeneralizedTime` `init` from `internal` to `public`.