(rx-constituents): Change `anything' to call

(rx-constituents): Change `anything' to call
rx-anything. Change `not-wordchar' assignment to "\\W" from
"[^[:word:]]".
(rx-group-if): New function.
(rx-parent): New variable.
(rx-and, rx-or): Put shy groups only when necessary.
(rx-bracket): Remove.
(rx-anything): New function.
(rx-any-delete-from-range, rx-any-condense-range)
(rx-check-any-string): New functions.
(rx-check-any): Return result as a list. Don't convert chars to
strings. Don't prepend "\\" to "^". Don't search for close
bracket. Check char category string. Call rx-form instead of
rx-to-string.
(rx-any): Rebuid to complete the function.
(rx-check-not): Fix char category regexp pattern string. Call
rx-form instead of rx-to-string.
(rx-not): Call rx-form instead of rx-to-string. Convert "[^]" to
"[^^]". Call regexp-quote for one char string when not called from
rx-not. Add "\\w", and toggle to upcase. Add the case of
"\\[SCBW]" to toggle.
(rx-=, rx->=, rx -**, rx-repeat, rx-submatch): Call rx-form
instead of rx-to-string.
(rx-kleene): Call rx-form instead of rx-to-string. Call
rx-group-if to adjust putting of shy groups.
(rx-atomic-p): Make check more precisely.
(rx-eval, rx-greedy): Call rx-form instead of rx-to-string.
(rx-regexp): Call rx-group-if.
(rx-form): New function.
(rx-to-string): Call rx-form, rx-group-if. Refine definition of
NO-GROUP.