Profpatsch 41e13149f7 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; }; }; }
```

2021-01-25 19:25:50 +01:00

8.8 KiB

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