Optimise `append` calls

Optimise `append` calls

Add the transforms

  (append) -> nil
  (append X) -> X
  (append '(X) Y) -> (cons 'X Y)
  (append (list X) Y) -> (cons X Y)
  (append (list X...) nil) -> (list X...)

and the argument transforms:

  (list X...) (list Y...) -> (list X... Y...)
  nil -> ;nothing
  CONST1 CONST2 -> CONST1++CONST2
  (list CONSTANTS...) -> '(CONSTANTS...)

(the last three for non-tail arguments only)

* lisp/emacs-lisp/byte-opt.el: New.