Add `GF2` type for Galois field elements

- 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