How to get instance member value from getSymbolsByUDA

[Remi Thebault]

Hi all,

I'm trying to establish a REST API by using the type system (used 
in both client and server code).

Considering the code

```d
struct Request
{
     Method method;
     string url;
     int apiLevel;
}

@Request(Method.GET, "/my-resource/%s", 1)
struct MyResourceGet
{
     @Param
     string name;

     // other members...
}

string requestUrl(ReqT)(ReqT req) if (isRequest!ReqT)
{
     import std.format : format;
     import std.traits : getSymbolsByUDA;

     Request reqAttr = RequestAttr!ReqT;

     alias paramSymbols = getSymbolsByUDA!(ReqT, Param);
     // return format(reqAttr.url, ????);
}

unittest
{
     MyResourceGet req;
     req.name = "thisone";
     assert(requestUrl(req) == "/my-resource/thisone");
}
```

In `requestUrl`, how do I actually get the value of `req.name` 
from `paramsSymbols` and `req`?