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/random_walk.py

@@ -0,0 +1,49 @@
+"""!
+@file random_walk.py
+@brief Turtle Challenge 4: generate a random walk.
+"""
+
+import random
+
+from turtle_challenges.colors import random_color
+from turtle_challenges.constants import (
+    FASTEST_SPEED,
+    WALK_HEADINGS,
+    WALK_PEN_SIZE,
+    WALK_STEP_DISTANCE,
+    WALK_STEPS,
+)
+from turtle_challenges.pen import Pen
+
+
+def random_walk(
+    pen: Pen,
+    steps: int = WALK_STEPS,
+    distance: float = WALK_STEP_DISTANCE,
+    rng: random.Random | None = None,
+) -> None:
+    """!
+    @brief Walk in random directions, changing color at every step.
+
+    Each step faces east, north, west or south at random and moves the same
+    distance in a new random RGB color.
+
+    @param pen The turtle that draws.
+    @param steps Number of steps to take.
+    @param distance Distance of one step, in turtle units.
+    @param rng Random generator for headings and colors; a shared one is used
+        if omitted.
+    @exception ValueError If @p steps is negative, or @p distance is not
+        positive.
+    """
+    if steps < 0:
+        raise ValueError(f"steps must not be negative, got {steps}")
+    if distance <= 0:
+        raise ValueError(f"distance must be positive, got {distance}")
+    generator = rng if rng is not None else random.Random()
+    pen.pensize(WALK_PEN_SIZE)
+    pen.speed(FASTEST_SPEED)
+    for _ in range(steps):
+        pen.color(random_color(generator))
+        pen.setheading(generator.choice(WALK_HEADINGS))
+        pen.forward(distance)