Aggressive refactoring

[jamesrswift]

When implementing logarithmic plots, and then when implementing polar plots, I ran into the challenge that the axis style has no control over how the data is plotted. This make sense if you treat axis styles as a purely visual element, however, this PR turns them into "drivers" that can change more behaviour of the underlying plot.

This is the initial PR, to allow people a better chance at collaborating on the refactor as well as to give maintainers a better chance at reviewing changes in smaller chunks rather than one hefty PR

Firstly, axis styles have been moved out of axes.typ (like in a previous draft PR), and instead are located in plot/axis-styles/. They become modules which must currently provide 3 functions: make-ctx (responsible for constructing the context within which data is prepared), data-viewport (responsible for transforming data along any number of axes, and onto the plot itself), and draw-axes (responsible for drawing the axes).

make-ctx now supplies an array axes rather than separately providing x and y, as a more general solution for when higher dimensional plots are implemented. It must also provide compute-fill-paths and compute-stroke-paths (previously supplied by the util.typ module) to allow for a finer grained control over clipping (of note: path computation occurs PRIOR to data-viewport transformations)

Plot elements (previously plot.add-<name>()) are now under plot.add.<name>, and plot.add has become plot.add.xy to disambiguate.

The school-book axis style remains unimplemented but I hope to bring it back at a later stage

Tracking can be found here: #28