Remove support for struct-based enums

[sdboyer]

Per #7 (comment), it makes no sense to support deriving structs from enums. Let's yank all the code that does it.

[ryantxu]

IIUC, this would mean use something like:

AType: "foo" | "bar" | *"baz" @cuetsy(targetType="enum")

Any ideas how/where we should add the comments/docs on what each element means?

[sdboyer]

#12 shows has examples of how i imagine this working instead.

Hmm...comments. Yeah. All i can immediately think of is creating a convention whereby the comment on the CUE type can be prefixed with the member name, and we have to just parse it all out. Not great, but passable, i think.

[sdboyer]

Updated the sample in #14 to indicate how i think comments could work