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/eval/src/value/arbitrary.rs

@@ -91,7 +91,7 @@ fn leaf_value() -> impl Strategy<Value = Value> {
         any::<bool>().prop_map(Bool),
         any::<i64>().prop_map(Integer),
         any::<f64>().prop_map(Float),
-        any::<NixString>().prop_map(String),
+        any::<Box<NixString>>().prop_map(String),
         any::<OsString>().prop_map(|s| Path(PathBuf::from(s).into_boxed_path())),
     ]
 }