dired-filter-by-predicate variables

Hi,

This is just a placeholder for discussion, don't pull this yet it's not completely ready.

1. What do you think of the helpers in dired-utils? I kinda like them but they also feel stupid. Do you think they are worth it?
2. Why did you say it makes more sense to define them in the `cond` of `dired-filter--make-filter-1`? To me it makes more sense inside the filter because that's the only place where it's usable anyway.
3. Would it make sense for `size` to use `f-size` from `f.el` so we get the full size of the directory for directories? If no, maybe add another variable `dsize` which would equal 0 for files?
4. It was difficult to test because I can't `C-x C-e` my way into redefining the filter, any idea how to achieve that without restarting emacs (maybe this one is just PEBKAC)?
5. Did you know about  `dired-re-dir` and friends? Maybe it makes sense to use them instead