yacc.c: fix warnings about integral types

Reported by Derek Clegg.
http://lists.gnu.org/archive/html/bug-bison/2018-10/msg00018.html

Rather than adding casts, we should be more careful with types.  For
instance yystate should be a yytype_int16.  But currently we can't: it
is also used sometimes for storing other things that state numbers.

* data/yacc.c (yyparse): Add missing casts.

TODO

@@ -1,10 +1,20 @@
 * Short term
+** yacc.c
+Now that ylwrap is fixed, we should include foo.tab.h from foo.tab.c rather
+than duplicating it.
+
 ** C++
 Move to int everywhere instead of unsigned?  stack_size, etc.  The parser
 itself uses int (for yylen for instance), yet stack is based on size_t.
 
 Maybe locations should also move to ints.
 
+** C
+Introduce state_type rather than spreading yytype_int16 everywhere?
+
+** glr.c
+yyspaceLeft should probably be a pointer diff.
+
 ** Graphviz display code thoughts
 The code for the --graph option is over two files: print_graph, and
 graphviz. This is because Bison used to also produce VCG graphs, but since