d: put YYEMPTY in the TokenKind

* data/skeletons/d.m4, data/skeletons/lalr1.d (b4_token_enums): Rename
YYTokenType as TokenKind.
Define YYEMPTY.
* examples/d/calc.y, tests/calc.at, tests/scanner.at: Adjust.

tests/calc.at

@@ -306,13 +306,13 @@ class CalcLexer(R) : Lexer
 
     // Handle EOF.
     if (input.empty)
-      return YYTokenType.EOF;
+      return TokenKind.CALC_EOF;
 
     // Numbers.
     if (input.front.isNumber)
       {
         semanticVal_.ival = parseInt;
-        return YYTokenType.NUM;
+        return TokenKind.NUM;
       }
 
     // Individual characters