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/nixexpr.hh

@@ -21,6 +21,13 @@ MakeError(TypeError, EvalError);
 MakeError(UndefinedVarError, Error);
 MakeError(MissingArgumentError, EvalError);
 
+class InfiniteRecursionError : public EvalError
+{
+    friend class EvalState;
+public:
+    using EvalError::EvalError;
+};
+
 /**
  * Position objects.
  */