Optional references

This library allows the template parameter T to be of reference type: T&, and to some extent, T const&.

However, since references are not real objects some restrictions apply and some operations are not available in this case:

Converting constructors
Converting assignment
InPlace construction
InPlace assignment
Value-access via pointer

Also, even though optional<T&> treats it wrapped pseudo-object much as a real value, a true real reference is stored so aliasing will ocurr:

Copies of optional<T&> will copy the references but all these references will nonetheless refer to the same object.
Value-access will actually provide access to the referenced object rather than the reference itself.

	Warning
	On compilers that do not conform to Standard C++ rules of reference binding, operations on optional references might give adverse results: rather than binding a reference to a designated object they may create an unexpected temporary and bind to it. For more details see Dependencies and Portability section.

Rvalue references and lvalue references to const have the ability in C++ to extend the life time of a temporary they bind to. Optional references do not have this capability, therefore to avoid surprising effects it is not possible to initialize an optional references from a temporary. Optional rvalue references are disabled altogether. Also, the initialization and assignment of an optional reference to const from rvalue reference is disabled.

const int& i = 1;            // legal
optional<const int&> oi = 1; // illegal