feat(tvix/eval): rm NixContext::join, add take_context & IntoIterator

In places where we want to extend context with that from another
NixString, use take_context() to split it off, then call .extend(),
making use of IntoIterator to avoid a bunch of clones.

Change-Id: I2460141a3ed776c64c36132b2203b6a1d710b922
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11705
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: edef <edef@edef.eu>

tvix/glue/src/builtins/derivation.rs

@@ -372,12 +372,12 @@ pub(crate) mod derivation_builtins {
                             return Ok(val);
                         }
 
-                        let (val_json, mut context) = match val.into_contextful_json(&co).await? {
+                        let (val_json, context) = match val.into_contextful_json(&co).await? {
                             Ok(v) => v,
                             Err(cek) => return Ok(Value::from(cek)),
                         };
 
-                        input_context = input_context.join(&mut context);
+                        input_context.extend(context.into_iter());
 
                         // No need to check for dups, we only iterate over every attribute name once
                         structured_attrs.insert(arg_name.to_owned(), val_json);