Add `rawFractionalSeconds` property to `GeneralizedTime`

[clintonpi]

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 public init to GeneralizedTime that accepts rawFractionalSeconds instead of fractionalSeconds and generates the latter 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.
• Document that _fractionalSeconds is a cached value and _rawFractionalSeconds is the source of truth for the numerical fractional seconds. (This holds directly for the new GeneralizedTime init that takes rawFractionalSeconds. And it holds indirectly for the existing init that takes fractionalSeconds, as no information is lost in the 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.

[Lukasa]

@swift-server-bot add to allowlist