This is the base class for some mathematical paths which map an integer
position '$n' to and from coordinates '$x,$y' in the 2D plane.

The current classes include the following. The intention is that any
'Math::PlanePath::Something' is a PlanePath, and supporting base classes or
related things are further down like 'Math::PlanePath::Base::Xyzzy'.

SquareSpiral four-sided spiral
PyramidSpiral square base pyramid
TriangleSpiral equilateral triangle spiral
TriangleSpiralSkewed equilateral skewed for compactness
DiamondSpiral four-sided spiral, looping faster
PentSpiral five-sided spiral
PentSpiralSkewed five-sided spiral, compact
HexSpiral six-sided spiral
HexSpiralSkewed six-sided spiral skewed for compactness
HeptSpiralSkewed seven-sided spiral, compact
AnvilSpiral anvil shape
OctagramSpiral eight pointed star
KnightSpiral an infinite knight's tour
CretanLabyrinth 7-circuit extended infinitely