[Nickle]New 'enum' union member type

[Keith Packard]

Borrowing from ML, Bart and I added a new union member 'type' called 
'enum'.  This type is the same as void, except that the constructor takes 
no argument (it doesn't need one in any case, 'void' has but a single 
value).

Also changed is the syntax for calling the union constructor, instead of 
the strange cast-like syntax:

	(<type> . <name>) value

we now have:

	<type> . <name> (value)

for regular union members.  For the new 'enum' members, we've got:

	<type> . <name>

They're called 'enum' because you can treat them a lot like an enumerated 
type:

	typedef union { int a; enum empty; enum broken; } foo;

	foo x;

	x = foo.a (10);		/* regular constructor */
	x = foo.empty;		/* enum constructor */
	union switch (x) {
	case a:
		printf ("a %d\n", x.a);
		break;
	case empty:
		printf ("empty\n");
		break;
	case broken:
		printf("broken\n");
		break;
	}

For syntactic sugar, you can create a union full of only enums with:

	typedef enum { red, green, blue, yellow } colors;

Now you can do things like:

	colors	c;

	c = colors.red;
	if (c == colors.yellow) printf ("yellow\n");

Of course, you can still use the union switch:

	union switch (c) {
	case red:
	case green:
	case blue:
	case yellow:
	}

Perhaps this final case should be changed to 'enum switch' instead?

-keith