ww: add Hare-style is/as postfix ops on tagged unions

`e is T` returns bool (variant tag == T's index); `e as T` unwraps
to T or exit(1) on mismatch. Postfix, same precedence as `:` cast.
TK_IS / N_TYPETEST / N_TYPEASSERT appended at the tail of their
enums so every prior numeric value stays unchanged — the
990_selfhost wwdump-diff stays byte-clean.

Cgen mirrors the match-case slot-based load (tag at +0, value at
+8/+16), so an N_IDENT tagged-union local works just like a
match scrutinee. Selfhost cgen inlines the slot resolution
because the wwstage cgen drops sign bits on `*i32` output
parameters in this position.

Renames `errors.is` -> `errors.equal` (the only naming collision;
the existing comment already noted it shared shape with
strings.equal/bytes.equal).

cmd/wcc/parse.c

@@ -670,6 +670,22 @@ parsepostfix(Parser *p, Node *lhs)
 			lhs = n;
 			break;
 		}
+		case TK_AS:
+		case TK_IS: {
+			/* Hare-style:
+			 *   e as T  — assert lhs is variant T of its tagged
+			 *             union; abort if not. Yields T.
+			 *   e is T  — bool: does lhs currently hold variant T?
+			 * Postfix, same level as `:` cast. */
+			Tkind k = p->cur.kind;
+			advance(p);
+			Node *n = newnode(p->a,
+			    k == TK_AS ? N_TYPEASSERT : N_TYPETEST, pp);
+			n->lhs = lhs;
+			n->rhs = parsetype(p);
+			lhs = n;
+			break;
+		}
 		default:
 			return lhs;
 		}