Add random_walk with tests

200 steps of equal length, each in a random heading of 0, 90, 180 or 270 degrees and
a random RGB color, with a thick line and the fastest speed.

Task: MIL-004#1

src/turtle_challenges/__init__.py

@@ -4,13 +4,15 @@
 100 Days of Code.
 
 The challenges so far are @ref turtle_challenges.square.draw_square,
-@ref turtle_challenges.dashed_line.draw_dashed_line and
-@ref turtle_challenges.shapes.draw_shape. Each takes the turtle as its
+@ref turtle_challenges.dashed_line.draw_dashed_line,
+@ref turtle_challenges.shapes.draw_shape and
+@ref turtle_challenges.random_walk.random_walk. Each takes the turtle as its
 first argument, so it can be tested without a window.
 """
 
 from turtle_challenges.colors import random_color, random_palette_color
 from turtle_challenges.dashed_line import draw_dashed_line
+from turtle_challenges.random_walk import random_walk
 from turtle_challenges.shapes import draw_shape, draw_shapes
 from turtle_challenges.square import draw_square
 
@@ -21,4 +23,5 @@ __all__ = [
     "draw_square",
     "random_color",
     "random_palette_color",
+    "random_walk",
 ]