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/vm/generators.rs

@@ -436,7 +436,7 @@ where
                                 })
                                 .with_span(&span, self)?;
 
-                            message = VMResponse::Value(Value::String(content.into()))
+                            message = VMResponse::Value(content.into())
                         }
 
                         VMRequest::PathExists(path) => {
@@ -607,7 +607,7 @@ pub async fn request_string_coerce(
     kind: CoercionKind,
 ) -> Result<NixString, CatchableErrorKind> {
     match val {
-        Value::String(s) => Ok(s),
+        Value::String(s) => Ok(*s),
         _ => match co.yield_(VMRequest::StringCoerce(val, kind)).await {
             VMResponse::Value(Value::Catchable(c)) => Err(c),
             VMResponse::Value(value) => Ok(value