Implement saving/loading spreadsheets to/from xlsx

[rokostik]

This PR implements spreadsheet <-> xlsx using the excelize library.

When saving the types are correctly saved in the xlsx.

When loading the fist sheet is used to load (should we parametrize this?). The loaded spreadsheet values are all strings. The library has the option to get a cell type but it's a bit cumbersome to use (it needs to be done for each cell in the excel cell index, eg. "A1". I can of course change it but I'm wondering if combining the current implementation with |autotype is good enough?

I've also added a mktmp function to the os context for easier testing.

[refaktor]

Great! I think if the spreadsheet has type information, an expected behavior would be that the types are transferred to the Rye spreadsheets and vice versa. If everything is just string loose information from xlsx and then we try to guess it again based on the format, which can cause rare but because of it even more unexpected bahaviour.

In Rye spreadsheet each cell is also typed separately (there is no per-column type information) so converting spreadsheets cell values to rye values of appropriate type makes sense to me.

[rokostik]

so converting spreadsheets cell values to rye values of appropriate type makes sense to me.

Noted, I'll prepare another PR for that change.