feat(tvix/eval): implement builtins.toJSON

Implements `Serialize` for `tvix_eval::Value`. Special care is taken
with serialisation of attribute sets, and forcing of thunks.

The tests should cover both cases well.

Change-Id: I9bb135bacf6f87bc6bd0bd88cef0a42308e6c335
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7803
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.exp

@@ -0,0 +1 @@
+"[42,\"hello\",13.37,[],[1,2,3],{},{\"name\":\"foo\",\"value\":42},{\"foo\":42}]"

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-literals.nix

@@ -0,0 +1,11 @@
+# tests serialisation of literal data
+builtins.toJSON [
+  42
+  "hello"
+  13.37
+  [ ]
+  [ 1 2 3 ]
+  { }
+  { name = "foo"; value = 42; }
+  { foo = 42; }
+]

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.exp

@@ -0,0 +1 @@
+"[42,42,\"42\"]"

tvix/eval/src/tests/tvix_tests/eval-okay-builtins-tojson-thunks.nix

@@ -0,0 +1,9 @@
+let
+  a = b * 2;
+  b = 21;
+in
+builtins.toJSON [
+  a
+  ((n: n * 2) 21)
+  (builtins.toJSON a)
+]