Draw arcs as arrows

[epost]

We'll want something like a function edge or arc that does or invokes something along the lines of this thing:
https://github.com/statebox/purescript-stbx-sandbox/blob/208b9e571f46b3c6814656e04f4d52f0356efb9c/src/PetrinetView.purs#L174-L178

There are various ways of defining SVG 'components', e.g. defs, which might help.

[antonleviathan]

Summary of porting Bert's code to create SVG arrows for petri nets.

placeLinePoint and transitionLinePoint are used for drawing different length lines from tx to place and vice-versa: github.com/statebox/elm-petrinet-viewer/blob/master/src/Data/LinearAlgebra.elm

This is where they're actually used to draw some arcs: https://github.com/statebox/elm-petrinet-viewer/blob/master/src/View/PetriNet.elm#L74

In trying to port this I partially succeeded but had to use an additional function to get the arrows to properly align:
https://github.com/statebox/purescript-stbx-sandbox/pull/6/files#diff-9d0e14d27aaddd1c5285228cf7113985R148

[antonleviathan]

@epost Can this be closed?