Consider adding value_or_eval()

[martinmoene]

See issue 54 at TartanLlama/optional.

Boost.Optional:

    template <typename F>
    value_type value_or_eval ( F f ) const&
      {
        if (this->is_initialized())
          return get();
        else
          return f();
      }
      
    template <typename F>
    value_type value_or_eval ( F f ) &&
      {
        if (this->is_initialized())
          return boost::move(get());
        else
          return f();
      }