feat(tvix/eval): implement getContext primop

Change-Id: I2c5068a28f9883a01b0ff80a5e5ab32ba18bfc1a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10437
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/value/string.rs

@@ -326,6 +326,10 @@ impl NixString {
         return self.1.as_mut();
     }
 
+    pub fn iter_context(&self) -> impl Iterator<Item = &NixContext> {
+        return self.1.iter();
+    }
+
     pub fn iter_plain(&self) -> impl Iterator<Item = &str> {
         return self.1.iter().flat_map(|context| context.iter_plain());
     }