Restore ambiguous value printer for nix-instantiate

The Nix team has requested that this output format remain unchanged.
I've added a warning to the man page explaining that `nix-instantiate
--eval` output will not parse correctly in many situations.

src/libexpr/print-ambiguous.hh

@@ -0,0 +1,24 @@
+#pragma once
+
+#include "value.hh"
+
+namespace nix {
+
+/**
+ * Print a value in the deprecated format used by `nix-instantiate --eval` and
+ * `nix-env` (for manifests).
+ *
+ * This output can't be changed because it's part of the `nix-instantiate` API,
+ * but it produces ambiguous output; unevaluated thunks and lambdas (and a few
+ * other types) are printed as Nix path syntax like `<CODE>`.
+ *
+ * See: https://github.com/NixOS/nix/issues/9730
+ */
+void printAmbiguous(
+    Value &v,
+    const SymbolTable &symbols,
+    std::ostream &str,
+    std::set<const void *> *seen,
+    int depth);
+
+}