Add a new operation on the interface port, CURRENT-EXPRESSION-CONTEXT,

Add a new operation on the interface port, CURRENT-EXPRESSION-CONTEXT,
that returns a context description associated with the expression most
recently read from the port.  The context description is a symbol
which says where the expression came from.  This information will be
used by the 6.001 code to make the generation of special definition
messages sensitive to the context.