Convert shapes from boolean to enum

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Switch from specifying the border shape from a boolean—true for round (elliptical/circular) and false for non-round (rectangular/square)—to an enum—explicitly specifying the shape by name.

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See more-shapes branch.

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6f503ac Convert from Boolean to enum shape specification

Convert from specifying border shape with a Boolean (true for round, elliptic, or circular borders and false for square or rectangular borders) to specifying border shape using the Shape enum (with RECTANGULAR for square and rectangular worlds and ELLIPTIC for circular and elliptic borders).

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Built on by #2.

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In pre-release version 2.0.0-dev-20w29a.