Thoughts about fdef utils?

Just wanted to stir up a conversation about this. I personally find specing functions to be tedious for most, simple functions.

Assume we had this spec:

```clojure
(ds/spec ::user {::id spec/uuid?
::age spec/pos-int?
::name spec/string?})
```

Assume we want to spec this function:

```clojure
(defn teenager?
[user]
(<= 13 (::age user) 19))
```

It would be neat if we could somehow spec this function easier

```clojure
(ds/specf teenager?
[::user] ; args
spec/boolean? ; ret
nil ; fn
)
```

Or even with a Schema like macro:

```clojure
(ds/defn teenager? :- spec/boolean?
[user :- ::user]
(<= 13 (::age user) 19))
```

I haven't actually fleshed out any of my thoughts, just wanted to see what you thought about some function specing utilities.

Start by reviewing the proposed ds/specf and ds/defn forms in this issue alongside the existing spec-tools API. Determine the intended scope and semantics for function argument, return, and optional function specs before implementation; the issue does not yet define a concrete done condition.

Written by the indexing model from the issue text.