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/scan-code.h

@@ -23,6 +23,7 @@
 
 # include "location.h"
 # include "named-ref.h"
+# include "uniqstr.h"
 
 struct symbol_list;
 
@@ -81,8 +82,11 @@ typedef struct code_props {
   /** \c NULL iff \c code_props::kind is not \c CODE_PROPS_RULE_ACTION.  */
   struct symbol_list *rule;
 
-  /* Named reference. */
+  /** Named reference. */
   named_ref *named_ref;
+
+  /** Type, for mid-rule actions.  */
+  uniqstr type;
 } code_props;
 
 /**
@@ -103,7 +107,8 @@ void code_props_none_init (code_props *self);
     /* .is_predicate = */ false,                \
     /* .is_used = */ false,                     \
     /* .rule = */ NULL,                         \
-    /* .named_ref = */ NULL                     \
+    /* .named_ref = */ NULL,                    \
+    /* .type = */ NULL,                         \
   }
 
 /** Initialized by \c CODE_PROPS_NONE_INIT with no further modification.  */
@@ -158,7 +163,8 @@ void code_props_symbol_action_init (code_props *self, char const *code,
  */
 void code_props_rule_action_init (code_props *self, char const *code,
                                   location code_loc, struct symbol_list *rule,
-                                  named_ref *name, bool is_predicate);
+                                  named_ref *name, uniqstr type,
+                                  bool is_predicate);
 
 /**
  * \pre