feat(tvix/eval): context-aware coerce_to_string

I am still undecided whether we need a CoercionKind to control
the coerced context, here's a simple attempt.

Change-Id: Ibe59d09ef26c519a6acfdfe392014446646dd6d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10426
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: raitobezarius <tvl@lahfa.xyz>

tvix/eval/src/value/string.rs

@@ -322,6 +322,10 @@ impl NixString {
         Self::new_context_from(context, &s.into_boxed_str())
     }
 
+    pub(crate) fn context_mut(&mut self) -> Option<&mut NixContext> {
+        return self.1.as_mut();
+    }
+
     pub fn iter_plain(&self) -> impl Iterator<Item = &str> {
         return self.1.iter().flat_map(|context| context.iter_plain());
     }