Derive Selectable should derive Queryable by default

[Elrendio]

Hey!

Recently I've been migrating Stockly's code base to use #[derive(Selectable)]. I'm not aware of use cases where a structure can implement Selectable without implementing Queryable. Therefore I think the Selectable should by default add a Queryable implementation with an optout as #[diesel(no_impl_queryable)].

Pros:

• More ergonomics, less characters to type
• Less cryptiques errors when you've forgotten a Queryable implementation. The most problematique part about this is that the error doesn't happen when you add the Selectable derive but when you try to use the struct.

Cons:

• This is a breaking change and can therefore be released only in diesel 3 if there's a version 3.
• This hides more the different concepts

[weiznich]

I think its reasonable to go that way, but I really don't want to do another major release soon.

What we could do without breaking change is to introduce another derive that combines Queryable + Selectable+ deprecate the using derive Queryable+Selectable` on the same struct.