fix(tvix/eval): make tvix display values like nix-instantiate(1)

In order for the test suite we have currently to be comparable to C++
Nix, we need to display values in the same way. This was largely the
case except in some weird cases.

* <CODE> for thunks and <CYCLE> for repeated thunks (?) are already in
  use. <CODE> formatting is tested by the oracle test suite already.

* Instead of lambda, we need to use <LAMBDA>

* <<primop>> and <<primop-app>> (a formatting C++ Nix uses nowhere)
  now are <PRIMOP> and <PRIMOP-APP>.

We'll probably want to have a fancier display of values (in a separate
trait) down the line. This could be used for interactive usage, e.g. the
REPL or a potential debugger.

There is a peculiarity with C++ Nix 2.3 formatting primops: import is
considered a <<PRIMOP-APP>>, since it is internally implemented by means
of scopedImport. This implementation detail no longer leaks in C++ Nix
2.13 nor in Tvix.

<CYCLE> display is untested at the moment, since we exhibit a
discrepancy to C++ Nix 2.3. Our current detection is more similar to C++
Nix 2.13—luckily it is also the more consistent of the two. See also
b/245.

Change-Id: I1d534434b02e470bf5475b3758920ea81e3420dc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8760
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/verify-lang-tests/default.nix

@@ -59,6 +59,10 @@ let
     "eval-okay-getattrpos-functionargs.nix" = [ nix ];
     # groupBy appeared (long) after 2.3
     "eval-okay-builtins-groupby-thunk.nix" = [ nix ];
+    # import is no longer considered a curried primop in Nix > 2.3
+    "eval-okay-import-display.nix" = [ nix ];
+    # Cycle detection and formatting changed sometime after Nix 2.3
+    "eval-okay-cycle-display-cpp-nix-2.13.nix" = [ nix ];
   };
 
   runCppNixLangTests = cpp-nix: