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

src/lalib/__init__.py

@@ -4,10 +4,26 @@ First, verify that your installation of `lalib` works:
 >>> import lalib
 >>> lalib.__version__ != '0.0.0'
 True
+
+`lalib` exposes its very own "words" (i.e., its public API) at the root
+of the package. They can be imported all at once with:
+
+>>> from lalib import *
+
+In addition to Python's built-in numbers, `lalib` comes with a couple of
+specific numeric data types, for example, `one` and `zero` representing
+the two elements of the Galois field `GF2`:
+
+>>> one + one
+zero
+>>> one + zero
+one
 """
 
 from importlib import metadata
 
+from lalib.elements import gf2
+
 
 try:
     pkg_info = metadata.metadata(__name__)
@@ -24,4 +40,15 @@ else:
     del pkg_info
 
 
+GF2, one, zero = gf2.GF2, gf2.one, gf2.zero
+
+
+del gf2
 del metadata
+
+
+__all__ = (
+    "GF2",
+    "one",
+    "zero",
+)

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__)