Deprecate old CIF enum declaration syntax

The CIF grammar contains:

     | ENUMKW Identifier EQTK CUROPENTK Identifiers CURCLOSETK SEMICOLTK
     | ENUMKW Identifier EQTK Identifiers SEMICOLTK

The former is the old syntax, the latter the new syntax. Previously enums had a set of literals and the order did not matter. Now the order does matter, and two enum types are only compatible if they have the same literals with the same names in the same order. The old {...} syntax variant makes it seem like a set, and thus that the order doesn't matter. We should deprecate the old syntax. We should generate a warning (e.g. 'Variant with curly brackets is deprecated.') if the old syntax is used. For backward compatibility, we won't remove it (for now).