[save] Stream Worksheet XML to Disk

[JanMarvin]

This is an experimental branch, using a custom C++ function to stream the XML file to disk. It is working, but not extensively tested and might still lack features. It has shown that it can reduce the memory consumption significantly.

[JanMarvin]

Cherry pick 15daf92

[JanMarvin]

Using this, is a game changer memory wise. Writing a 500,000 x 100 rnorm data frame previously required - with the optimizations from this branch - ~28GB. Writing the output file with pugixml using a custom xml writer, reduces the required memory amount by 12-14GB.
The changes in the branch skip a few guards that check that the cells and rows are in the correct order.

options("openxlsx2.export_with_pugi" = FALSE)

This branch skips building the XML file in memory before it is written to the disk. The custom XML writer simply flushes what is available into a text file. Without any checks for correct XML etc.

[JanMarvin]

wb_save(flush = TRUE) is now possible