bug#58278: Add new function seq-keep

[Lars Ingebrigtsen]

Robert Pluim <rpluim@gmail.com> writes:

> Thatʼs why I said "apart from the syntactic sugar" 😀

Having to rewrite

(seq-keep #'cl-digit-char-p '(?6 ?a))
=> (6)

to

(cl-mapcan (lambda (c)
             (let ((res (cl-digit-char-p c)))
               (and res (list res))))
           '(?6 ?a))
=> (6)

is more than "syntactic sugar" in my book.  Nobody would want to write
code like the latter.