C++ FFI templates

[mantielero]

I think you are right. One thing that I have observed is that the 
[manual](https://nim-lang.org/docs/manual.html#importcpp-pragma-importcpp-for-objects)
 says: 
    
    
    type
      StdMap {.importcpp: "std::map", header: "<map>".} [K, V] = object
    
    
    Run

It looks like this syntax is discouraged. And now the compiler suggest things 
like: 
    
    
    type
      StdMap [K,V] {.importcpp: "std::map", header: "<map>".} = object
    
    
    Run

The manual should be updated.