feat: Implement RichText support for XLSX

[FlixCoder]

For #440 .

I implemented a RichText representation and changed the Data representation to use it. The parsing is so far only implemented for XLSX, the other formats continue to only ever produce Data::String even if it is actually rich text content.

The shared strings also have to be rich text now ^^

I think it turned out quite well, let me know what you think :)

[tafia]

Awesome, this looks really neat!
I made a few tiny comments, the most important one being of not having a DataRef::RichText variant.

[FlixCoder]

Adjusted to the requested changes. I also found a few more little things that I missed, e.g. the DataType::get_string implementation for Data and such.

[tafia]

Thanks a lot.
I think I still don't see the reason not to keep DataRef::SharedString and just add a new variant DataRef::SharedRichText if necessary.

[tafia]

I think it is wrong to force all strings to be RichText. There are already some situation where memory is an issue.

[FlixCoder]

Do you have an idea how to handle this? We need to store either normal strings or rich text, both can be the case at any shared string. So naturally you would make an enum:

enum SharedString {
    String(String),
    RichText(RichText),
}

But then the enum type is at least as big as RichText, if not more. So using &[SharedString] does not solve the memory issue.

So how are we able to store rich text while keeping the memory layout of string in the usual case? We could make the rich text varian RichText(Box<RichText>), but that has different overhead, that I am not sure if it is better than additional memory use?

[tafia]

I'd expect a SharedString and SharedRichText ??
What is the issue exactly to force RichText all over the place?

[FlixCoder]

I can't really progress until #441 (comment) . Could you elaborate a bit how you imagine things to be?