Organize top-level imports for lalib

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

tests/test_top_level_imports.py

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