c++: give public access to the symbol kind

symbol_type::token () was removed: it returned the token kind of a
symbol.  To do that, one needs to convert from the symbol kind to the
token kind, which requires a table.

This broke some users' unit tests for scanners, see
https://lists.gnu.org/r/bug-bison/2020-01/msg00001.html
https://lists.gnu.org/r/bug-bison/2020-03/msg00020.html
https://lists.gnu.org/r/help-bison/2020-04/msg00005.html

Instead of making this possible again, let's check the symbol's kind
instead.  So give proper access to a symbol's kind.

That feature existed, undocumented, as 'type_get()'.  Let's rename
this as 'kind()'.

* data/skeletons/c++.m4, data/skeletons/glr.cc,
* data/skeletons/lalr1.cc (type_get): Rename as...
(kind): This.
(type_get): Install a backward compatibility alias.
* doc/bison.texi (Complete Symbols): Document symbol_type and
symbol_type::kind.

doc/bison.texi

@@ -12193,6 +12193,17 @@ location.  Invocations of @samp{%lex-param @{@var{type1} @var{arg1}@}} yield
 additional arguments.
 @end deftypefun
 
+@defcv {Type} {parser} {symbol_type}
+A ``complete symbol'', that binds together its kind, value and (when
+applicable) location.
+@end defcv
+
+@deftypemethod {symbol_type} {symbol_kind_type} kind () @code{const}
+The kind of this symbol.
+@end deftypemethod
+
+@sp 1
+
 For each token kind, Bison generates named constructors as follows.
 
 @deftypeop  {Constructor} {parser::symbol_type} {} {symbol_type} (@code{int} @var{token}, @code{const @var{value_type}&} @var{value}, @code{const location_type&} @var{location})