fix(tvix/eval): implement cppnix JSON-serialisation semantics

This drops the usage of serde::Serialize, as the trait can not be used
to implement the correct semantics (function colouring!).

Instead, a manual JSON serialisation function is written which
correctly handles toString, outPath and other similar weirdnesses.

Unexpectedly, the eval-okay-tojson test from the C++ Nix test suite
now passes, too.

This fixes an issue where serialising data structures containing
derivations to JSON would fail.

Change-Id: I5c39e3d8356ee93a07eda481410f88610f6dd9f8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8209
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/to_xml.rs

@@ -126,7 +126,8 @@ fn value_variant_to_xml<W: Write>(w: &mut EventWriter<W>, value: &Value) -> Resu
         Value::AttrNotFound
         | Value::Blueprint(_)
         | Value::DeferredUpvalue(_)
-        | Value::UnresolvedPath(_) => {
+        | Value::UnresolvedPath(_)
+        | Value::Json(_) => {
             return Err(ErrorKind::TvixBug {
                 msg: "internal value variant encountered in builtins.toXML",
                 metadata: Some(Rc::new(value.clone())),