mv include/ruby/{3,impl}

Devs do not love "3".

include/ruby/impl/stdbool.h

@@ -0,0 +1,51 @@
+#ifndef RUBY3_STDBOOL_H                              /*-*-C++-*-vi:se ft=cpp:*/
+#define RUBY3_STDBOOL_H
+/**
+ * @file
+ * @author     Ruby developers <ruby-core@ruby-lang.org>
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ * @warning    Symbols   prefixed   with   either  `RUBY3`   or   `ruby3`   are
+ *             implementation details.   Don't take  them as canon.  They could
+ *             rapidly appear then vanish.  The name (path) of this header file
+ *             is also an  implementation detail.  Do not expect  it to persist
+ *             at the place it is now.  Developers are free to move it anywhere
+ *             anytime at will.
+ * @note       To  ruby-core:  remember  that   this  header  can  be  possibly
+ *             recursively included  from extension  libraries written  in C++.
+ *             Do not  expect for  instance `__VA_ARGS__` is  always available.
+ *             We assume C99  for ruby itself but we don't  assume languages of
+ *             extension libraries. They could be written in C++98.
+ * @brief      C99 shim for <stdbool.h>
+ */
+#include "ruby/3/config.h"
+
+#if defined(__bool_true_false_are_defined)
+# /* Take that. */
+
+#elif defined(__cplusplus)
+# /* bool is a keyword in C++. */
+# if defined(HAVE_STDBOOL_H) && (__cplusplus >= 201103L)
+#  include <cstdbool>
+# endif
+#
+# ifndef __bool_true_false_are_defined
+#  define __bool_true_false_are_defined
+# endif
+
+#elif defined(HAVE_STDBOOL_H)
+# /* Take stdbool.h definition. */
+# include <stdbool.h>
+
+#else
+typedef unsigned char _Bool;
+# /* See also http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2229.htm */
+# define bool  _Bool
+# define true  ((_Bool)+1)
+# define false ((_Bool)+0)
+# define __bool_true_false_are_defined
+#endif
+
+#endif /* RUBY3_STDBOOL_H */