[spec] Add specs for spec forms

In the current patch, keywords referencing specs are missing except for keys in `s/keys`. What would the right way to model the keywords? One way would be to add `qualified-keyword?` as fourth option to the or-spec of `::spec`. Than `::spec` could be used as spec for the spec arg in `s/valid?`, `s/conform`, `s/form` and others.

Yeah, that would probably be good.

Another observation: Currently s/and and s/or are allowed to contain no preds:

there should be either a clear semantic definition what an empty form will mean or we should just use s/+ here and require at least one pred. For the and-form, we could define that (s/and) is the same as (s/and any?) and the same as just any?. For the or-form it is difficult, because s/conform returns tagged results. So conforming any value with (s/or) will result in ::s/invalid which renders an empty or-form useless.

There is a clear semantic definition for these - they are the same as clojure.core/and and clojure.core/or. As far as I'm concerned, this is all correct.

Thanks. I see. The documentation in clojure.core defines that (clojure.core/and) returns true and (clojure.core/or) returns nil.