Suggestion: add a lazy_vector<T> example

First, thanks for creating yap! I am trying to extend the lazy_vector example to create a `lazy_vector` version. This would need another template parameter for the `lazy_vector_expr`
struct, like

```
template
struct lazy_vector_expr
{
static const boost::yap::expr_kind kind = Kind;

Tuple elements;

// Note that this does not return an expression; it is greedily evaluated.
I operator[] (std::size_t n) const;
};
```
However, this doesn't work with the provided macros `BOOST_YAP_USER_BINARY_OPERATOR`. Does it mean i should implement a different version of `BOOST_YAP_USER_BINARY_OPERATOR`? What is the best way to achieve this?