lookup macro could be simpler

[davepacheco]

The lookup_resource! macro used to generate:

struct Foo<'a> {
    key: FooKey<'a>,
}

enum FooKey<'a> {
    Name(...),
    PrimaryKey(...),
    ...
}

There's no need for two types here -- the top-level Foo type could be the enum. The current behavior is a relic of when it used to have more data.

[davepacheco]

I'm not so sure this is a great idea. Although it does simplify things, it means the enum and its variants are now exposed outside the lookup macro and module.

[plotnick]

I like this change very much; I always found it a bit confusing and slightly un-ergonomic having a separate type for the key. I leave it to you of course, if you think it's not a good idea, but to me the code reads significantly better.

[david-crespo]

I am also in favor