Pipe tables ftw

Change to rendering pipe tables when formatting Markdown documents. We put lots of effort into supporting Pandoc's very clever multi-line table layout, but when trying to make changes to these tables it proved almost impossible to get the alignment and formatting right. In practice, everyone uses pipe tables regardless, and although it's a huge drag that you don't get line wrapping in these passages it is nevertheless better from a diff standpoint. So, switch to pipe table output.

There's also better escaping for [, ], and _ characters; this is slightly annoying in that it introduces is more visual noise into the Markdown and isn't strictly necessary if a link annotation [...](htps://..) isn't present, it matches the behaviour of Pandoc itself and reduces ambiguity for the parser inbound.

Builds with GHC 8.10.4 and also on MacBooks now.

Back to the Inkscape

We had a good run with rsvg-convert but it was always a bit barebones and no surprise that it's fallen behind. No reason not to rely on inkscape's command-line export processing features. As we did before. What's old is new.

Change to LuaLaTeX

Clearly this one is worthy of a release note. We've changed the back-end to LuaLaTeX. It would seem that although what we were using previously, XeLaTeX, was excellent at modern font handling and UTF-8, it had problems with more modern constructs like dates and times. Deep breath, time to join the future.