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/eval/src/value/mod.rs

@@ -338,8 +338,8 @@ impl Value {
             let coerced: Result<BString, _> = match (value, kind) {
                 // coercions that are always done
                 (Value::String(mut s), _) => {
-                    if let Some(ctx) = s.context_mut() {
-                        context = context.join(ctx);
+                    if let Some(ctx) = s.take_context() {
+                        context.extend(ctx.into_iter());
                     }
                     Ok((*s).into())
                 }