Improve SVG stringification

[staticintlucas]

• Improve SVG path output
  • Is 3 decimal digits enough? With 1000 dots-per-key this will have a resolution of 19.05 nm, while with 5 digits we have 0.1905 nm. Either seems excessive TBH (implemented in c17fd05)
  • Improve stringification, for example a line currently produces "l10 -50" while we could do "l10-50". printf supports the "% f" format which inserts a ' ' or '-', but rust doesn't seem to have an equivalent (implemented in c17fd05)
  • Currently we use absolute moves and relative everything else, but in some cases the opposite would produce smaller output. Is it worth implementing some logic to optimise this?
• Have an option to toggle between pretty printed and compressed output? (see also: Allow for custom formatting bodoni/svg#68)
  • Could we generate pretty output and then use a library to compress the output? How inefficient is this approach?