yacc: use the most appropriate integral type for state numbers

Currently we properly use the "best" integral type for tables,
including those storing state numbers.  However the variables for
state numbers used in yyparse (and its dependencies such as
yy_stack_print) still use int16_t invariably.  As a consequence, very
large models overflow these variables.

Let's use the "best" type for these variables too.  It turns out that
we can still use 16 bits for twice larger automata: stick to unsigned
types.

However using 'unsigned' when 16 bits are not enough is troublesome
and generates tons of warnings about signedness issues.  Instead,
let's use 'int'.

Reported by Tom Kramer.
https://lists.gnu.org/archive/html/bug-bison/2019-09/msg00018.html

* data/skeletons/yacc.c (b4_state_num_type): New.
(yy_state_num): Be computed from YYNSTATES.
* tests/linear: New.
* tests/torture.at (State number type): New.
Use it.

THANKS

@@ -179,6 +179,7 @@ Tim Landscheidt           tim@tim-landscheidt.de
 Tim Van Holder            tim.van.holder@pandora.be
 Tobias Frost              tobi@debian.org
 Todd Freed                todd.freed@gmail.com
+Tom Kramer                kramer@nist.gov
 Tom Lane                  tgl@sss.pgh.pa.us
 Tom Tromey                tromey@cygnus.com
 Tomasz Kłoczko            kloczko.tomasz@gmail.com