d: use the SymbolType enum for symbol kinds

* data/skeletons/d.m4 (b4_symbol_enum, b4_declare_symbol_enum): New.
* data/skeletons/lalr1.d: Use them.
Use SymbolType, SymbolType.YYSYMBOL_YYEMPTY etc. where appropriate.
(undef_token_, token_number_type, yy_error_token_): Remove.

data/skeletons/d.m4

@@ -148,9 +148,9 @@ private static immutable b4_int_type_for([$2])[[]] yy$1_ =
 ])
 
 
-## ------------------------- ##
-## Assigning token numbers.  ##
-## ------------------------- ##
+## -------------------------- ##
+## (External) token numbers.  ##
+## -------------------------- ##
 
 # b4_token_enum(TOKEN-NAME, TOKEN-NUMBER)
 # ---------------------------------------
@@ -159,8 +159,8 @@ m4_define([b4_token_enum],
 [b4_token_format([  %s = %s,
 ], [$1])])
 
-# b4_token_enums(LIST-OF-PAIRS-TOKEN-NAME-TOKEN-NUMBER)
-# -----------------------------------------------------
+# b4_token_enums
+# --------------
 # Output the definition of the tokens as enums.
 m4_define([b4_token_enums],
 [/* Tokens.  */
@@ -172,6 +172,39 @@ b4_symbol_foreach([b4_token_enum])
 }
 ])
 
+## --------------------------- ##
+## (Internal) symbol numbers.  ##
+## --------------------------- ##
+
+
+# b4_symbol_enum(SYMBOL-NUM)
+# --------------------------
+# Output the definition of this symbol as an enum.
+m4_define([b4_symbol_enum],
+[m4_ifval(b4_symbol([$1], [sid]),
+         [m4_format([[%s = %s]],
+                    b4_symbol([$1], [sid]),
+                    b4_symbol([$1], [number]))])])
+
+
+# b4_declare_symbol_enum
+# ----------------------
+# The definition of the symbol internal numbers as an enum.
+# Defining YYEMPTY here is important: it forces the compiler
+# to use a signed type, which matters for yytoken.
+m4_define([b4_declare_symbol_enum],
+[[  /* Symbol type.  */
+  public enum SymbolType
+  {
+    ]m4_join([,
+    ],
+             ]b4_symbol_sid([-2])[ = -2,
+             b4_symbol_map([b4_symbol_enum]))[
+  };
+]])])
+
+
+
 # b4-case(ID, CODE)
 # -----------------
 m4_define([b4_case], [    case $1: