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/serde/src/de.rs

@@ -91,7 +91,8 @@ impl<'de> de::Deserializer<'de> for NixDeserializer {
             | Value::AttrNotFound
             | Value::Blueprint(_)
             | Value::DeferredUpvalue(_)
-            | Value::UnresolvedPath(_) => Err(Error::Unserializable {
+            | Value::UnresolvedPath(_)
+            | Value::Json(_) => Err(Error::Unserializable {
                 value_type: self.value.type_of(),
             }),
         }