Add tests for LaserCodeRegistry, clean up.

* Store a deque rather than an iterator so it can be pickled
* Remove mangling from staticmethod (and rename now that it's no longer
  a generator)
* Rename "get" to "alloc" to make the mutation clear
* Move to its own package (the changes I'm working on make this no
  longer mission generator specific)
* Remove useless exception class. It's never caught so the unique type
  isn't needed

tests/lasercodes/test_lasercoderegistry.py

@@ -0,0 +1,15 @@
+from game.lasercodes.lasercoderegistry import LaserCodeRegistry
+
+
+def test_initial_laser_codes() -> None:
+    reg = LaserCodeRegistry()
+    assert list(reg.available_codes)[:5] == [1688, 1687, 1686, 1685, 1684]
+    assert list(reg.available_codes)[-5:] == [1715, 1714, 1713, 1712, 1711]
+    assert len(reg.available_codes) == 192
+
+
+def test_alloc_laser_code() -> None:
+    reg = LaserCodeRegistry()
+    assert reg.alloc_laser_code() == 1688
+    assert 1688 not in reg.available_codes
+    assert len(reg.available_codes) == 191