Make `lalib.elements.galois.to_gf2()` a detail

- `to_gf2()` is only to be used within `GF2Element.__new__()`
  => rename it into `_to_gf2()`
- the test cases in `TestGF2ConstructorWithCastedValue`
  cover everything in `TestGF2Casting`
  => retire the redundant test cases
     and make the test suite run faster