set past end of stream, set Byte to -1, Char and Code to end_of_file, at end of stream (#479)

src/prolog/machine/streams.rs

@@ -59,7 +59,7 @@ pub enum EOFAction {
     Reset,
 }
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum AtEndOfStream {
     Not,
     At,
@@ -328,6 +328,12 @@ impl Stream {
         self.stream_inst.0.borrow_mut().0
     }
 
+    #[inline]
+    pub(crate)
+    fn at_end_of_stream(&mut self) -> bool {
+        self.position_relative_to_end() == AtEndOfStream::At
+    }
+
     #[inline]
     pub(crate)
     fn set_past_end_of_stream(&mut self) {