encode black holes as tApp values

checking for isBlackhole in the forceValue hot path is rather more
expensive than necessary, and with a little bit of trickery we can move
such handling into the isApp case. small performance benefit, but under
some circumstances we've seen 2% improvement as well.

〉 nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'

before:

  Time (mean ± σ):      4.429 s ±  0.002 s    [User: 3.929 s, System: 0.500 s]
  Range (min … max):    4.427 s …  4.433 s    10 runs

after:

  Time (mean ± σ):      4.396 s ±  0.002 s    [User: 3.894 s, System: 0.501 s]
  Range (min … max):    4.393 s …  4.399 s    10 runs

src/libexpr/eval.hh

@@ -77,6 +77,14 @@ struct PrimOp
      */
     std::optional<ExperimentalFeature> experimentalFeature;
 
+    /**
+     * Whether to hide this primop in diagnostics.
+     *
+     * Used to hide the fact that black holes are primop applications from
+     * stack traces.
+     */
+    bool hideInDiagnostics;
+
     /**
      * Validity check to be performed by functions that introduce primops,
      * such as RegisterPrimOp() and Value::mkPrimOp().