This also simplifies the beta checking API by unifying it under
objects (makes much more sense that way) and reworks the validator
to have betaness support within its context state, allowing checks
to be done easily in any place.
The betaness checks are disabled for types for the time being,
because otherwise there are too many errors (types are assumed
to be stable as they are not tagged beta, but they reference beta
classes all over the place). Set EOLIAN_TYPEDECL_BETA_WARN to 1
in your environment to force enable the checks.