nix-build/nix-shell: Eliminate call to nix-instantiate / nix-store

Note that this removes the need for a derivation symlink, so the
--drv-path and --add-drv-link flags now do nothing.

src/libexpr/eval.hh

@@ -118,6 +118,8 @@ public:
     Expr * parseExprFromString(const string & s, const Path & basePath, StaticEnv & staticEnv);
     Expr * parseExprFromString(const string & s, const Path & basePath);
 
+    Expr * parseStdin();
+
     /* Evaluate an expression read from the given file to normal
        form. */
     void evalFile(const Path & path, Value & v);