consume whitespace before reading a term in loader.pl, modify prolog_load_context at file key to better reflect loading context, use it when reporting singleton variables (#812)

src/machine/streams.rs

@@ -395,6 +395,7 @@ impl Stream {
             StreamInstance::TlsStream(..) |
             StreamInstance::ReadlineStream(..) |
             StreamInstance::StaticStr(..) |
+            StreamInstance::PausedPrologStream(..) |
             StreamInstance::Bytes(..) => Some(0),
             _ => None,
         };