defconst for `custom-printers'.

[Luc Teirlinck]

I believe that the following defconst should clearly be changed to a
defvar.  I will do so unless there are objections.  The `nil' value is
_intended_ to be changed.  The defconst produces a compiler warning.
This is one of these variables where just using `setq' has no effect,
but functions need to be called.

Maybe this one should actually even be made into a defcustom with a :set
function.  Maybe people who use this variable could comment on that.

Here is the defconst:

(defvar custom-printers nil
  ;; e.g. '((symbolp . pkg::print-symbol))
  "An alist for custom printing of any type.
Pairs are of the form (PREDICATE . PRINTER).  If PREDICATE is true
for an object, then PRINTER is called with the object.
PRINTER should print to `standard-output' using cust-print-original-princ
if the standard printer is sufficient, or cust-print-prin for complex things.
The PRINTER should return the object being printed.

Don't modify this variable directly.  Use `add-custom-printer' and
`delete-custom-printer'")