Clean up "magic string" cell color in Game of Life

[NicMcPhee]

In the Game of Life exercises, I used the "magic string" "blue" for the color of my circles, which then propagates out across the tests, etc., in an awkward way.

One option would be to add another named constant for that color instead of scattering the "magic string" "blue" all over the code.

Alternatively I could write a draw-cell function that encapsulates both "solid" and "blue" so they're only in one place.

[NicMcPhee]

Or I could just define a variable cell that is the (circle cell-radius "solid" "blue") logic, which would probably be nicer than a draw-cell function. (Credit to Jackson Lennox for that idea.)