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/value/thunk.rs

@@ -25,8 +25,6 @@ use std::{
     rc::Rc,
 };
 
-use serde::Serialize;
-
 use crate::{
     errors::ErrorKind,
     spans::LightSpan,
@@ -269,15 +267,6 @@ impl TotalDisplay for Thunk {
     }
 }
 
-impl Serialize for Thunk {
-    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
-    where
-        S: serde::Serializer,
-    {
-        self.value().serialize(serializer)
-    }
-}
-
 /// A wrapper type for tracking which thunks have already been seen in a
 /// context. This is necessary for cycle detection.
 ///