feat(tvix/eval): teach builtins.toXML context

XmlEmitter gains a NixContext field, and `write_typed_value` extends it
with all context elements present in the passed value.

Once all serialization is done, a into_context() function returns the
collected context, so we can construct a NixString with context.

Tests for this live in tvix-glue, as we use builtins.derivation, which
is not present in the tvix-eval crate.

Fixes b/398.

Change-Id: I85feaaa17b753885f8a017a54e419ec4e602af21
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11704
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Alyssa Ross <hi@alyssa.is>

tvix/eval/src/builtins/mod.rs

@@ -1504,8 +1504,19 @@ mod pure_builtins {
         }
 
         let mut buf: Vec<u8> = vec![];
-        to_xml::value_to_xml(&mut buf, &value)?;
-        Ok(buf.into())
+        let context = to_xml::value_to_xml(&mut buf, &value)?;
+
+        Ok((
+            buf,
+            // FUTUREWORK: We have a distinction between an empty context, and
+            // no context at all. Fix this.
+            if !context.is_empty() {
+                Some(Box::new(context))
+            } else {
+                None
+            },
+        )
+            .into())
     }
 
     #[builtin("placeholder")]