An efficient built-in mapcan

An efficient built-in mapcan

A built-in version of `mapcan' avoids consing up (and GC'ing) the
intermediate list.

* src/fns.c (Fmapcan): New built-in.
(syms_of_fns): Define.

* lisp/emacs-lisp/cl.el (mapcan): Remove defalias.

* lisp/emacs-lisp/cl-extra.el (cl-mapcan): Use built-in `mapcan'
if only one sequence is provided.

* lisp/progmodes/hideif.el (hif-delimit):
* lisp/dired-aux.el (dired-do-find-regexp):
* lisp/woman.el (woman-parse-colon-path): Use `mapcan' instead of
`cl-mapcan'.

* lisp/woman.el (eval-when-compile): Require 'cl-lib only when
compiling.

* lisp/mouse.el (mouse-buffer-menu-map):
* lisp/net/pop3.el (pop3-uidl-dele):
* lisp/progmodes/gud.el (gud-jdb-build-source-files-list):
* lisp/cedet/semantic/db-find.el (semanticdb-fast-strip-find-results):
* lisp/cedet/semantic/symref/grep.el (semantic-symref-derive-find-filepatterns):
* lisp/gnus/nnmail.el (nnmail-split-it):
* lisp/gnus/gnus-sum.el (gnus-articles-in-thread):
* lisp/gnus/gnus-registry.el (gnus-registry-sort-addresses):
* lisp/gnus/gnus-util.el (gnus-mapcar): Use `mapcan'.