[RFC] vibe.web.web: Support JSON more natively

[wilzbach]

A couple of improvement ideas to help working with Json:

• allow Json as function parameter -> call with req.json (-> vibe.web.web: Allow Json as a auto-injected parameter type #1853)
• allows to return strings -> req.writeBody (for those simple APIs where "OK" is needed) (-> vibe.web.web: Allow to return strings #1854)
• This PR:
  • by default an unknown return type is serialized to Json (no need to yield a compile-time error)
  • allow auto-deserialization of Json parameters (this is only attempted if there's a json response and then only for the first parameter)

[s-ludwig]

The direction definitely makes sense. What I had in mind for quite a while now was to let the REST and web interface generators share more code - maybe in the form of using RestInterface!T also for web interfaces (with a few adjustments). This means that specifying a whole-body JSON parameter in web interfaces would work the same as for RESTful interfaces: @bodyParam("p") void fun(Json p) {}.

One reason for going that route (apart from strengthening the least-surprise factor) is that I recently had a lot of places where having @queryParam and @bodyParam in the web interface would have been really useful.

[wilzbach]

The direction definitely makes sense.

As this stalled a bit in the queue, I have split this PR into three pieces which should make it a lot easier to evaluate them in small pieces (I think the split-offs are non-controversial, only this one might be).

This means that specifying a whole-body JSON parameter in web interfaces would work the same as for RESTful interfaces: @bodyParam("p") void fun(Json p) {}.

I agree that this wouldn't be bad, but Json is so commonly nowadays that I would prefer void fun(Json p){} (#1853) or even auto-deserialization from Json: void fun(MyStruct myStruct) (this PR)

[wilzbach]

This means that specifying a whole-body JSON parameter in web interfaces would work the same as for RESTful interfaces: @bodyParam("p") void fun(Json p) {}.

FYI: this might soon be void fun(@bodyParam Json p){} - the respective language change has already been approved and a working PR is in the queue (dlang/dmd#7576). It just needs a few final tweaks, so if you want to I could with-hold the param changes and make this PR, s.t. it's just about allowing to return a arbitrary type in vibe.web.web which in turn will be serialized to Json.