Hide `gf2` sub-classes even more

- the `gf2` sub-classes `GF2One` and `GF2Zero` have no purpose
  other than to provide unique `help(one)` and `help(zero)` messages
- delete them at the bottom of `lalib.elements.galois`
  to prevent them from being imported at all
  (`type(one)` and `type(zero)` still appear to be the `gf2` class,
   which is a little white lie, provided by some meta class)
- differentiate between official and inofficial API in the test suite