feat: Walk on spheres

[keenancrane]

This PR adds a new example walk-on-spheres. See the README for details.

[codecov]

Codecov Report

Merging #1022 (ac212c1) into main (1a00e4c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1022   +/-   ##
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  Coverage   64.14%   64.14%           
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  Files          62       62           
  Lines        7945     7945           
  Branches     1802     1802           
=======================================
  Hits         5096     5096           
  Misses       2733     2733           
  Partials      116      116           

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[samestep]

This is incredible, thanks so much for making this @keenancrane! Possibly our most compelling Penrose example yet.

[samestep]

Unicode, nice 😎

[samestep]

This is just a matter of aesthetics, but could we do this instead? (same for the other .sub files in this directory)

Suggested change

	Step x0, x1, x2, x3, x4
	x1 := sampleBoundary( x0 )
	x2 := sampleBoundary( x1 )
	x3 := sampleBoundary( x2 )
	x4 := sampleBoundary( x3 )
	Step x0
	Step x1 := sampleBoundary( x0 )
	Step x2 := sampleBoundary( x1 )
	Step x3 := sampleBoundary( x2 )
	Step x4 := sampleBoundary( x3 )

[joshsunshine]

@samestep I think we should merge this now. I don't think your comment is significant enough to delay.

[keenancrane]

@samestep Yeah, I can see an argument for that coding style also. I think ultimately how this should look is some kind of array syntax:

Step x[5]
for i = 1, .., length(x)
   x[i] = sampleBoundary( x[i-1] )

Of course, that doesn't exist yet! It may also be useful for people to see that declaring all your variables up front is possible as one way to write Substance code. So, I'm gonna leave it for now… (and squash/merge). Thanks!