bool conversion for record references (see also #477)

I decided to allow for an 'unbound' reference to allow
default construction of elements involving record references.

I am aware of the implications, but I place the focus
on the value nature of GenNode elements; the RecordRef
was introduced only as a means to cary out diff comparisons
and similar computations.

src/lib/diff/record.hpp

@@ -428,6 +428,12 @@ namespace diff{
       // standard copy operations acceptable
       
       
+      explicit
+      operator bool()  const
+        {
+          return bool(record_);
+        }
+      
       /** target is accessed by cast
        * @throws error::Logic on bottom reference
        */

tests/library/diff/generic-record-representation-test.cpp

@@ -306,7 +306,7 @@ namespace test{
           RecordRef<string> empty;
           CHECK (bool(empty) == false);
           CHECK (nullptr == empty.get());
-          VERIFY_ERROR (BOTTOM_VALUE, RecS& (empty));
+          VERIFY_ERROR (BOTTOM_VALUE, empty.operator RecS&() );
           
           RecordRef<string> ref(oo);
           CHECK (ref);