Warn about comparing quoted lists (etc) using `eq`

[Mattias Engdegård]

I'm about to push a new byte-compiler warning that finds mistakes like

  (eq x '(ho hum))

Ie, attempts to compare, by identity, literals that may not match anything at 
all: quoted lists, strings, floats etc.

The warning finds about 20 such mistakes in the Emacs tree, including two by 
this programmer. So far no false positives have turned up, although not all 
warnings indicate actual bugs (many things just happen to work by luck). 
Typically the remedy is to use equal instead of eq, and so on.

The warning applies to arguments in calls to eq, eql, memq, memql, assq, rassq, 
remq and delq.

Given its usefulness and low risk (it does not actually change the behaviour of 
anything) I suggest it be added to the emacs-29 branch. Objections? (Clearly 
we'd like to fix the mistakes found in emacs-29, and doing so is a lot more 
convenient if the warning is actually there too.)

Patch below for reference. I don't think it merits a NEWS entry but could 
certainly add one.

0001-Warn-about-unmatchable-constant-args-to-eq-memq-etc.patch
Description: Binary data