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/primops.cc

@@ -4263,6 +4263,29 @@ static RegisterPrimOp primop_splitVersion({
 });
 
 
+static void prim_blackHoleFn(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    state.error("infinite recursion encountered")
+        .debugThrow<InfiniteRecursionError>();
+}
+
+static PrimOp primop_blackHole = {
+    .name = "«blackHole»",
+    .args = {},
+    .fun = prim_blackHoleFn,
+    .hideInDiagnostics = true,
+};
+
+static Value makeBlackHole()
+{
+    Value v;
+    v.mkPrimOp(&primop_blackHole);
+    return v;
+}
+
+Value prim_blackHole = makeBlackHole();
+
+
 /*************************************************************
  * Primop registration
  *************************************************************/