Please include NOTF

[phoe at teknik.io]

After brief discussion on #lisp, I hereby suggest a macro for inclusion in Alexandria. 

The macro in question is NOTF.

## Reason:

An example code (from my own project) for negating the value of a place is:
(setf (left-now-p (walk-parent shape))
      (not (left-now-p (walk-parent shape))))
This code is problematic because a) it causes double evaluation of place subforms, b) it's long and repetitive.

I would rather write:
(notf (left-now-p (walk-parent shape)))

Lisp has modify macros like INCF/DECF for numbers but does not have such a macro for booleans. It seems like a trivial utility, but also one that might be easily overlooked.

## Upsides:

The place is evaluated only once.
The resulting code is shorter.

## Downsides

None that I can see at the moment.

## Code:

;; Please forgive me - I am not used to making PRs by mail.

(export 'notf :alexandria)

(define-modify-macro notf () not
  "Sets the value of PLACE to its logical negation and returns the new value.")

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BR
Michal "phoe" Herda