allow serializing/deserializing BsonObject to/from a D struct

[timotheecour]

(as in json via orange library, etc)

adapting from the example in https://github.com/2night/mondo/blob/master/source/bsond.d :

BsonObject obj = 
   BO
   (
      "type", "place", 
      "address", 
      BO
      (
         "street", "main st.", 
         "number", 15
      ), 
      "tags", BA("first", "middle", "last"),
      "array", [10, 20, 30] 
   );

struct Bar{
  string street;
  int number;
}

struct Foo{
  string type;
  Bar address;
  string[]tags;
  int[]array;
}

Foo foo;
deserialize(foo, obj);
assert(foo.type=="place");
assert(serialize(foo)==obj);

it shall throw on mixed type arrays for example.