accomodate \0\ in partial strings, print null as \0\ (#267, #526), update prolog parser, version bump

src/prolog/machine/system_calls.rs

@@ -1110,6 +1110,11 @@ impl MachineState {
 
                 let h = self.heap.h();
 
+                if atom.as_str().is_empty() {
+                    self.fail = true;
+                    return Ok(());
+                }
+
                 let pstr = self.heap.allocate_pstr(atom.as_str());
                 let pstr_tail = self.heap[h + 1].as_addr(h + 1);