New rx implementation

* lisp/emacs-lisp/rx.el:
* test/lisp/emacs-lisp/rx-tests.el:
* doc/lispref/searching.texi (Rx Constructs):
Rewrite rx for correctness, clarity, and performance.  The new
implementation retains full compatibility and has more comprehensive
tests.

* lisp/emacs-lisp/re-builder.el (reb-rx-font-lock-keywords):
Adapt to changes in internal variables in rx.el.

doc/lispref/searching.texi

@@ -1044,11 +1044,9 @@ customisation.
 
 The various forms in @code{rx} regexps are described below.  The
 shorthand @var{rx} represents any @code{rx} form, and @var{rx}@dots{}
-means one or more @code{rx} forms.  Where the corresponding string
+means zero or more @code{rx} forms.  Where the corresponding string
 regexp syntax is given, @var{A}, @var{B}, @dots{} are string regexp
 subexpressions.
-@c With the new implementation of rx, this can be changed from
-@c 'one or more' to 'zero or more'.
 
 @subsubheading Literals
 

lisp/emacs-lisp/re-builder.el

@@ -816,13 +816,12 @@ If SUBEXP is non-nil mark only the corresponding sub-expressions."
 
 (require 'rx)
 (defconst reb-rx-font-lock-keywords
-  (let ((constituents (mapcar (lambda (rec)
-                                (symbol-name (car rec)))
-                              rx-constituents))
-        (syntax (mapcar (lambda (rec) (symbol-name (car rec))) rx-syntax))
+  (let ((constituents (mapcar #'symbol-name rx--builtin-forms))
+        (syntax (mapcar (lambda (rec) (symbol-name (car rec)))
+                        rx--syntax-codes))
         (categories (mapcar (lambda (rec)
                               (symbol-name (car rec)))
-                            rx-categories)))
+                            rx--categories)))
     `(
       (,(concat "(" (regexp-opt (list "rx-to-string") t) "[[:space:]]")
        (1 font-lock-function-name-face))