Organize top-level imports for lalib.elements

- make `GF2`, `one`, and `zero`, defined in the `lalib.elements.gf2`
  module, available as top-level imports in the `lalib.elements`
  sub-package via `from lalib.elements import *`
- provide some code snippets in the sub-package's docstring
- test the star import

tests/elements/test_top_level_imports.py

@@ -0,0 +1,18 @@
+"""Test top-level imports for `lalib.elements`."""
+
+from lalib import elements as top_level
+
+
+def test_top_level_imports():
+    """Verify `from lalib.elements import *` works."""
+    environment = {}
+
+    exec("...", environment, environment)  # noqa: S102
+    defined_vars_before = set(environment)
+
+    exec("from lalib.elements import *", environment, environment)  # noqa: S102
+    defined_vars_after = set(environment)
+
+    new_vars = defined_vars_after - defined_vars_before
+
+    assert new_vars == set(top_level.__all__)

tests/test_docstrings.py

@@ -14,6 +14,7 @@ import xdoctest
     "module",
     [
         "lalib",
+        "lalib.elements",
         "lalib.elements.gf2",
     ],
 )