- add `GF2` class in the `lalib.elements` sub-package
implementing a typical Galois field with two elements
- the singleton objects `one` and `zero` are the concrete
instances of the `GF2` type for the end users
- besides the typical Galois arithmetic, `one` and `zero`
behave like the built-in numbers `1` and `0`
and implement the `numbers.Rational` interface
- add exhaustive docstrings with usage examples
- add (unit) test cases with 100% coverage
- use xdoctest to validate code snippets in docstrings
- make xdoctest part of the nox session "test" via
the new `test_docstrings()` test case
- add nox session "test-docstrings" for convenience;
also, `xdoctest.doctest_module()` does not discover
docstrings that are imported at the root of the package
=> each new module with docstrings must be added to
`test_docstrings()` by hand, which is likely forgotten
=> the nox session "test-docstrings" should run on CI
