* @enumTagName renamed to @tagName and it works on enums and
union-enums
* Remove the EnumTag type. Now there is only enum and union,
and the tag type of a union is always an enum.
* unions support specifying the tag enum type, and they support
inferring an enum tag type.
* Enums no longer support field types but they do support
setting the tag values. Likewise union-enums when inferring
an enum tag type support setting the tag values.
* It is now an error for enums and unions to have 0 fields.
* switch statements support union-enums
closes#618
see #383
there is a plan to unify most of the reflection into 2
builtin functions, as outlined in the above issue,
but this gives us needed features for now, and we can
iterate on the design in future commits
