lib/debug: add traceFnSeqN

Immensely helpful when you want to see the changes a function makes to
its value as it passes through.

Example:

```
$ nix-instantiate --strict --eval -E '(with import ./lib; traceFnSeqN 2 "id" (x: x) { a.b.c = 3; })'
trace: {
  fn = "id";
  from = {
    a = {
      b = {…};
    };
  };
  to = {
    a = {
      b = {…};
    };
  };
}
{ a = { b = { c = 3; }; }; }
```

lib/default.nix

@@ -130,7 +130,7 @@ let
       assertMsg assertOneOf;
     inherit (self.debug) addErrorContextToAttrs traceIf traceVal traceValFn
       traceXMLVal traceXMLValMarked traceSeq traceSeqN traceValSeq
-      traceValSeqFn traceValSeqN traceValSeqNFn traceShowVal
+      traceValSeqFn traceValSeqN traceValSeqNFn traceFnSeqN traceShowVal
       traceShowValMarked showVal traceCall traceCall2 traceCall3
       traceValIfNot runTests testAllTrue traceCallXml attrNamesToStr;
     inherit (self.misc) maybeEnv defaultMergeArg defaultMerge foldArgs