mode-line-eol-desc needs to use assoc, not assq

[Ami Fischman]

mode-line-eol-desc uses assq to look up desc in
mode-line-eol-desc-cache, but the key used is computed by
coding-system-eol-type which returns a vector sometimes.  This results
in mode-line-eol-desc-cache growing without bounds over the lifetime
of an emacs process.  Changing mode-line-eol-desc to use assoc instead
of assq (so that the vectors are compared with equal and so have a
chance of returning t) fixes this.

Repro:
$ emacs -Q
eval (length mode-line-eol-desc-cache)
do this many times, the value is always 1.
eval (prefer-coding-system 'utf-8-emacs)
eval (length mode-line-eol-desc-cache)
do this many times; the value keeps increasing by 1 on every call(!).
C-h v mode-line-eol-desc-cache RET
eval (length mode-line-eol-desc-cache)
witness the returned value increasing by 3 for every C-h v
(describe-variable) as above.

Many innocuous actions such as visiting files and creating buffers
will cause this variable to grow with many many duplicate entries.

Drive the variable arbitrarily high by evaling:

(progn
  (dotimes (j 1000)
    (switch-to-buffer "does-not-exist")
    (kill-buffer)
    (redisplay))
  (insert (format "%d " (length mode-line-eol-desc-cache))))

In an emacs running for several days I've seen the variable get over
420,000 entries in it, according to prin1-to-string.

Making the s/assq/assoc/ change as above fixes all these repro cases.

-a