Encapsulate accesses to IndexStore::streams and ::stream_aliases

These two fields are able to hold `Stream` instances, which predicates like `close/1`
expect to be managed properly for their correctness. To ensure that this is the case,
I have removed direct accesses to those two fields, so that they can be properly managed
in one place.

src/machine/loader.rs

@@ -1808,7 +1808,7 @@ impl Machine {
     pub(crate) fn push_load_context(&mut self) -> CallResult {
         let stream = self.machine_st.get_stream_or_alias(
             self.machine_st.registers[1],
-            &self.indices.stream_aliases,
+            &self.indices,
             atom!("$push_load_context"),
             2,
         )?;