add support for typed mid-rule actions

Prompted on Piotr Marcińczyk's message:
http://lists.gnu.org/archive/html/bug-bison/2017-06/msg00000.html.
See also http://lists.gnu.org/archive/html/bug-bison/2018-06/msg00001.html.

Because their type is unknown to Bison, the values of midrule actions are
not treated like the others: they don't have %printer and %destructor
support.  In addition, in C++, (Bison) variants cannot work properly.

Typed midrule actions address these issues.  Instead of:

    exp: { $<ival>$ = 1; } { $<ival>$ = 2; }   { $$ = $<ival>1 + $<ival>2; }

write:

    exp: <ival>{ $$ = 1; } <ival>{ $$ = 2; }   { $$ = $1 + $2; }

* src/scan-code.h, src/scan-code.l (code_props): Add a `type` field to
record the declared type of an action.
(code_props_rule_action_init): Add a type argument.
* src/parse-gram.y: Accept an optional type tag for actions.
* src/reader.h, src/reader.c (grammar_current_rule_action_append): Add
a type argument.
(grammar_midrule_action): When a mid-rule is typed, pass its type to
the defined dummy non terminal symbol.

src/reader.h

@@ -56,7 +56,7 @@ void grammar_current_rule_symbol_append (symbol *sym, location loc,
                                          named_ref *nref);
 /* Attach an ACTION to the current rule.  */
 void grammar_current_rule_action_append (const char *action, location loc,
-                                         named_ref *nref);
+                                         named_ref *nref, uniqstr tag);
 /* Attach a PREDICATE to the current rule.  */
 void grammar_current_rule_predicate_append (const char *predicate, location loc);
 void reader (void);