Rename `lalib.elements.gf2.GF2` & friends

- the future (concrete) Galois `Field` implementation
  shall receive the name `GF2` (as per common math notation)
  => name conflict with the current `GF2` class
     implementing the elements of the future Galois `Field`
  => rename the current `GF2` class into `GF2Element`
- because `GF2Element` is a bit tedius to type for the end user,
  we introduce a `gf2` alias in line with the naming convention
  for the built-in data types (e.g., `int` or `float`)
  that are also used as elements of (other) `Field`s
  => name conflict with the current `lalib.elements.gf2` module
  => rename the module into `lalib.elements.galois`
- adjust the docstrings to refer to "the `gf2` type"
- adjust the top-level imports and tests