bug#60196: 29.0.60; re-builder should read all forms for the rx syntax

[Mattias Engdegård]

The handling of rx input in re-builder is indeed unsatisfactory. We could adopt 
your proposed solution:

> Ideally, re-builder should allow users to simply write
> 
>     "<i>" (group (*? anychar)) "</i>"

which would be a definite improvement. We could also accept a Lisp expression 
that is evaluated, so that you'd write

    (rx "<i>" (group (*? anychar)) "</i>")

On the surface this looks more verbose, but since re-builder itself would 
provide the `(rx )` boilerplate, there wouldn't actually be any more typing. 
The main advantage would be that regexps could be built up from smaller pieces:

    (rx-let ((spaces (* " "))
             (identifier (: alpha (* alnum))))
      (rx identifier spaces "->" spaces identifier))

Either would be a definite improvement to the current rather strange re-builder 
behaviour with respect to rx input. Any preference?