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.hh

@@ -51,4 +51,10 @@ void prim_importNative(EvalState & state, const PosIdx pos, Value * * args, Valu
  */
 void prim_exec(EvalState & state, const PosIdx pos, Value * * args, Value & v);
 
+/**
+ *  Placeholder value for black holes, used to represent black holes as
+ *  applications of this value to the evaluated thunks.
+ */
+extern Value prim_blackHole;
+
 }