Merge #463

463: Adds ordinal direction unit vectors. r=astronouth7303 a=pathunstrom

Primary use case is to reduce the cognitive load in early lessons:

```python
class OurSprite(Sprite):
    def on_update():
       self.facing = Left
```

Co-authored-by: Piper Thunstrom <pathunstrom@gmail.com>

docs/reference/directions.rst

@@ -0,0 +1,8 @@
+.. py:currentmodule:: ppb.directions
+
+===============================
+Directions
+===============================
+
+.. automodule:: ppb.directions
+   :members:

docs/reference/index.rst

@@ -26,4 +26,5 @@ decisions are made, see the :doc:`/discussion/index` section.
    engine
    sound
    camera
+   directions
    features/index

ppb/__init__.py

@@ -12,12 +12,28 @@
 The classes, modules, and methods exported directly are the most used parts of
 the library and intended to be used by users at all levels (barring
 make_engine). Advanced features tend to be in their own modules and subpackages.
+
+Exports:
+
+* :class:`~ppb_vector.Vector`
+* :class:`BaseScene`
+* :class:`Circle`
+* :class:`Image`
+* :class:`Sprite`
+* :class:`Square`
+* :class:`Sound`
+* :class:`Triangle`
+* :mod:`events`
+* :class:`Font`
+* :class:`Text`
+* :mod:`directions`
 """
 
 import logging
 import warnings
 from typing import Callable
 
+from ppb import directions
 from ppb import events
 from ppb_vector import Vector
 from ppb.assets import Circle
@@ -34,7 +50,7 @@
 __all__ = (
     # Shortcuts
     'Vector', 'BaseScene', 'BaseSprite', 'Circle', 'Image', 'Sprite',
-    'Square', 'Sound', 'Triangle', 'events', 'Font', 'Text',
+    'Square', 'Sound', 'Triangle', 'events', 'Font', 'Text', 'directions',
     # Local stuff
     'run', 'make_engine',
 )
@@ -78,6 +94,7 @@ def run(setup: Callable[[BaseScene], None] = None, *, log_level=logging.WARNING,
 
     Sample usage:
     ::
+
        import ppb
 
        def setup(scene):
@@ -87,6 +104,7 @@ def setup(scene):
 
     Alternatively:
     ::
+
        import ppb
 
        class Game(ppb.BaseScene):

ppb/directions.py

@@ -0,0 +1,17 @@
+"""
+The ordinal directions.
+
+A collection of normalized vectors to be referenced by name.
+
+Best used for the positions or facings of :class:`Sprites <ppb.Sprite>`.
+"""
+from ppb_vector import Vector
+
+Up = Vector(0, 1).normalize()  #: Unit vector to the top of the screen from center.
+Down = Vector(0, -1).normalize()  #: Unit vector to the bottom of the screen from center.
+Left = Vector(-1, 0).normalize()  #: Unit vector to the left of the screen from center.
+Right = Vector(1, 0).normalize()  #: Unit vector to the right of the screen from center.
+UpAndLeft = (Up + Left).normalize()  #: Unit vector diagonally up and to the left of the screen from center.
+UpAndRight = (Up + Right).normalize()  #: Unit vector diagonally up and to the right of the screen from center.
+DownAndLeft = (Down + Left).normalize()  #: Unit vector diagonally down and to the left of the screen from center.
+DownAndRight = (Down + Right).normalize()  #: Unit vector diagonally down and to the right of the screen from center.