Allow complemented character class escapes within regex brackets.

The complement-class escapes \D, \S, \W are now allowed within
bracket expressions.  There is no semantic difficulty with doing
that, but the rather hokey macro-expansion-based implementation
previously used here couldn't cope.

Also, invent "word" as an allowed character class name, thus "\w"
is now equivalent to "[[:word:]]" outside brackets, or "[:word:]"
within brackets.  POSIX allows such implementation-specific
extensions, and the same name is used in e.g. bash.

One surprising compatibility issue this raises is that constructs
such as "[\w-_]" are now disallowed, as our documentation has always
said they should be: character classes can't be endpoints of a range.
Previously, because \w was just a macro for "[:alnum:]_", such a
construct was read as "[[:alnum:]_-_]", so it was accepted so long as
the character after "-" was numerically greater than or equal to "_".

Some implementation cleanup along the way:

* Remove the lexnest() hack, and in consequence clean up wordchrs()
to not interact with the lexer.

* Fix colorcomplement() to not be O(N^2) in the number of colors
involved.

* Get rid of useless-as-far-as-I-can-see calls of element()
on single-character character element names in brackpart().
element() always maps these to the character itself, and things
would be quite broken if it didn't --- should "[a]" match something
different than "a" does?  Besides, the shortcut path in brackpart()
wasn't doing this anyway, making it even more inconsistent.

Discussion: https://postgr.es/m/2845172.1613674385@sss.pgh.pa.us
Discussion: https://postgr.es/m/3220564.1613859619@sss.pgh.pa.us

src/include/regex/regguts.h

@@ -127,6 +127,18 @@
 #define ISBSET(uv, sn)	((uv)[(sn)/UBITS] & ((unsigned)1 << ((sn)%UBITS)))
 
 
+/*
+ * known character classes
+ */
+enum char_classes
+{
+	CC_ALNUM, CC_ALPHA, CC_ASCII, CC_BLANK, CC_CNTRL, CC_DIGIT, CC_GRAPH,
+	CC_LOWER, CC_PRINT, CC_PUNCT, CC_SPACE, CC_UPPER, CC_XDIGIT, CC_WORD
+};
+
+#define NUM_CCLASSES 14
+
+
 /*
  * As soon as possible, we map chrs into equivalence classes -- "colors" --
  * which are of much more manageable number.
@@ -164,12 +176,14 @@ struct colordesc
 #define  NOSUB	 COLORLESS		/* value of "sub" when no open subcolor */
 	struct arc *arcs;			/* chain of all arcs of this color */
 	chr			firstchr;		/* simple char first assigned to this color */
-	int			flags;			/* bit values defined next */
+	int			flags;			/* bitmask of the following flags: */
 #define  FREECOL 01				/* currently free */
 #define  PSEUDO  02				/* pseudocolor, no real chars */
-#define  UNUSEDCOLOR(cd) ((cd)->flags & FREECOL)
+#define  COLMARK 04				/* temporary marker used in some functions */
 };
 
+#define  UNUSEDCOLOR(cd) ((cd)->flags & FREECOL)
+
 /*
  * The color map itself
  *
@@ -199,8 +213,6 @@ struct colordesc
  * appear in increasing chr-value order.
  */
 
-#define NUM_CCLASSES 13			/* must match data in regc_locale.c */
-
 typedef struct colormaprange
 {
 	chr			cmin;			/* range represents cmin..cmax inclusive */