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/test_docstrings.py

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