Make BSONArrayBuilder/BSONObjBuilder behavior crash on common mis-uses

TEST(BSONObjBuilderTest, BadUseTest) {                                                                                                                                                           

    BSONObjBuilder root;                                                                                                                                                                      

    BSONArrayBuilder array1(root.subobjStart("arr1"));                                                                                                                                        

    array1.append(3);                                                                                                                                                                                                                                                                                                                                                 

    array1.doneFast();                                                                                                                                                                        

    BSONArrayBuilder array2(root.subobjStart("arr2"));                                                                                                                                        

    // Notice how we're using 'array1' here. It'd be nice if this tripped an invariant or something.                                                                                            

    array1.append(4);                                                                                                                                                                         

    array2.doneFast();                                                                                                                                                                        

    // This "just works" even though it doesn't produce the object you'd expect.                                                                                                                       

    auto obj = root.done();                                                                                                                                                                                                                                                                                             

    // ... later someone tries to use the object.                                                                                                                                             

    std::cout << obj << std::endl;                                                                                                                                                            

    // Kind of a strange object, and clearly not what was intended:                                                                                                                                    

    // { arr1: { 0: 3 }, arr2: { 1: 4 } }                                                                                                                                                     

}