refactor(tvix/eval): Box Value::String

NixString is *quite* large - like 80 bytes - because of the extra
capacity value for BString and because of the context. We want to keep
Value small since we're passing it around a lot, so let's box the
NixString inside Value::String to save on some memory, and make cloning
ostensibly a little cheaper

Change-Id: I343c8b4e7f61dc3dcbbaba4382efb3b3e5bbabb2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10729
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/serde/src/de.rs

@@ -347,7 +347,7 @@ impl<'de> de::Deserializer<'de> for NixDeserializer {
         if let Value::Attrs(attrs) = self.value {
             let mut map = MapDeserializer::new(attrs.into_iter().map(|(k, v)| {
                 (
-                    NixDeserializer::new(Value::String(k)),
+                    NixDeserializer::new(Value::from(k)),
                     NixDeserializer::new(v),
                 )
             }));

tvix/serde/src/de_tests.rs

@@ -222,7 +222,7 @@ mod test_builtins {
         match x {
             Value::String(s) => {
                 let new_string = NixString::from(format!("hello {}", s.to_str().unwrap()));
-                Ok(Value::String(new_string))
+                Ok(Value::from(new_string))
             }
             _ => Err(ErrorKind::TypeError {
                 expected: "string",