(fast-alist-clean! alist) is an alternative to fast-alist-clean that produces a normed result.
Logically this function is just fast-alist-clean; we leave it
enabled and would think it odd to ever prove a theorem about it.
Under the hood, this is the same as fast-alist-clean except that it
uses something like hons-acons! instead of hons-acons. You
generally should not use this function unless you really know what
you're doing and understand the drawbacks discussed in hons-acons!.