rebirth: Use cdr in libprebirth replacement proc definitions

This removes the need to use the ES `arguments' reference.  Not only is this
what we want to do, but it's _necessary_---the next commit will introduce
environments, and wrapping procedures in lambdas breaks the `arguments'
reference in each of these cases.

Note that fold and map can now be written in Rebirth Lisp, but I'm just
leaving them alone for now.

* build-aux/bootstrap/rebirth.scm
  (append, string-append, +, -, map): Use define cdr in place of ES
    `arguments'.

build-aux/bootstrap/rebirth.scm

@@ -240,8 +240,7 @@
 
    (define (es:arg->arr args)
      (string->es "Array.prototype.slice.call($$args)"))
-   (define (list)
-     (es:arg->arr (string->es "arguments")))
+   (define (list . xs) xs)
 
    ;; warning: only compares two values
    (define (= x y)
@@ -284,12 +283,12 @@
      (es:-assert-pair pair)
      (string->es "$$pair.slice(1)"))
 
-   (define (append)
+   (define (append . args)
      (fold (lambda (x xs)
              (es:-assert-list x)
              (string->es "$$xs.concat($$x)"))
            (list)
-           (es:arg->arr (string->es "arguments"))))
+           args))
 
    ;; warning: these two are wholly inadequate
    (define (list? xs)
@@ -307,26 +306,23 @@
      (string->es "typeof $$s1 === 'string' && $$s1 === $$s2"))
    (define (string-ref s i)
      (string->es "$$s[$$i] || $$error(`value out of range: ${$$i}`)"))
-   (define (string-append)
-     (let ((args (es:arg->arr (string->es "arguments"))))
-       (string->es "$$args.join('')")))
+   (define (string-append . xs)
+     (string->es "$$xs.join('')"))
 
    (define (eq? x y)
      (string->es "$$x === $$y"))
 
    ;; R7RS math
-   (define (+)
-     (let ((args (es:arg->arr (string->es "arguments"))))
+   (define (+ . xs)
      (fold (lambda (y x)
              (string->es "$$x + $$y"))
            0
-             args)))
-   (define (-)
-     (let ((args (es:arg->arr (string->es "arguments"))))
+           xs))
+   (define (- . xs)
      (fold (lambda (y x)
              (string->es "$$x - $$y"))
-             (car args)
-             (cdr args))))
+           (car xs)
+           (cdr xs)))
    (define (zero? x)
      (eq? x 0))
 
@@ -336,14 +332,9 @@
      (string->es "$$xs.reduce((prev, x) => $$f(x, prev), $$init)"))
 
    ;; warning: map here uses the length of the first list, not the shortest
-   ;; (we implement this in ES for now so that we don't have to augment
-   ;; Prebirth Lisp to support the "rest" procedure definition syntax)
-   (define (map f)
-     (string->es "__a = arguments")  ; because let introduces a function
-     (let* ((args (es:arg->arr (string->es "__a")))
-            (xs (cdr args)))
+   (define (map f . xs)
      (string->es
-        "$$xs[0].map((_, i) => $$f.apply(null, $$xs.map(x => x[i])))")))
+      "$$xs[0].map((_, i) => $$f.apply(null, $$xs.map(x => x[i])))"))
 
    (define (es:regexp s opts)
      (string->es "new RegExp($$s, $$opts)"))