'Null' literal (as opposed to 'Nothing' literal)

This idea addresses scenario #95.

A new `Null` literal in VB which always evaluates to either a null reference or a null nullable value.

# Advantages

Would be more intuitive. Addresses the problem without changing the semantics of Nothing or the If operator.

# Drawbacks

There's no way to enforce this at run-time because there's no difference between a null reference created by `Null` and a null reference created by `Nothing`. All conversion run-time operators will continue to convert a null reference to the default value of the target type.