Add experimental inline instrumented defn macro

[dvingo]

This came out of the discussion over here:
#695

When working in cljs having the code output by macros in the same namespace you're working in will enable repl-driven development without needing to think about refreshing the namespace where instrument! was called.

This PR adds a macro, inspired by Guardrails/Ghostwheel >defn macro, which will output an instrumented function.
A parallel noop ns can be added to allow controlling this via shadow-cljs config (see https://github.com/fulcrologic/guardrails#dead-code-elimination)

I needed to add the {:registry (m/default-schemas)} lines because the schemas used for the parsing code will not always be available if the user is using a custom registry.

[mauricioszabo]

So, it works but there are some issues with metadata:

:arglists is empty, and I tried to add a docstring but it also gets removed from metadata...

[dvingo]

thanks for trying it out (and checking for the meta)! - I will refactor that to support it. I think the >fn should be straightforward too, I'm interested to see that in action

[mauricioszabo]

Sure, please let me know if I can help in any way.

I tried some combinations of with-meta and others, and found a sad thing: seems that it's not possible to override the :arglists element using metadata. I looked at Prismatic Schema's implementation, and indeed it uses defn under the hood, so maybe it's the only way to make it work...

[mauricioszabo]

I found a way to keep the meta with minimum effort, if you want to use it:

Suggested change

	schema (as-> (map ->schema parglists) $ (if single (first $) (into [:function] $)))]
	`(def ~name (m/-instrument {:schema ~schema :report ~report}
	(c/fn ~name
	~@(map (fn [{:keys [arglist prepost body]}] `(~arglist ~prepost ~@body)) parglists)
	~@(when-not single (some->> arities val :meta vector))))))))
	schema (as-> (map ->schema parglists) $ (if single (first $) (into [:function] $)))
	meta-to-defn (cond-> meta doc (assoc :doc doc))]
	`(do
	(c/defn ~name ~meta-to-defn ~(map :arglist parglists))
	(set! ~name
	(m/-instrument {:schema ~schema :report ~report}
	(c/fn ~name
	~@(map (fn [{:keys [arglist prepost body]}] `(~arglist ~prepost ~@body)) parglists)
	~@(when-not single (some->> arities val :meta vector)))))))))

But this code does not work with Clojure (because you can't set! root bindings). I'm supposing ->defn is considered an implementation detail, so we can send an additional parameter (like is-clojurescript?) to handle the Clojure case?

[dvingo]

Thanks for looking into this! based on that I went with a strategy to produce an actual defn that just delegates to the instrumented version.

[dvingo]

This was implemented in #825