Generate a method when struct has list field

[jitsni]

http.idl has

struct HttpBeginEx
{
  list<http::HttpHeader> headers;
}

The generated builder has the following method.

    public Builder headers(Consumer<ListFW.Builder<HttpHeaderFW.Builder, HttpHeaderFW>> mutator) 

It would be convenient to have the following method:

    public Builder headers$item(Consumer<HttpHeaderFW.Builder> mutator) {
      headersRW.item(mutator);
      super.limit(headersRW.limit());
      return this;
    }