split internal.h into files

One day, I could not resist the way it was written.  I finally started
to make the code clean.  This changeset is the beginning of a series of
housekeeping commits.  It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits.  No
lines of codes are either added or removed, except the obvious file
headers/footers.  The generated binary is identical to the one before.

internal/util.h

@@ -0,0 +1,25 @@
+#ifndef INTERNAL_UTIL_H /* -*- C -*- */
+#define INTERNAL_UTIL_H
+/**
+ * @file
+ * @brief      Internal header corresponding util.c.
+ * @author     \@shyouhei
+ * @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    DO NOT ADD RANDOM GARBAGE HERE THIS FILE IS FOR util.c
+ */
+
+/* util.c */
+char *ruby_dtoa(double d_, int mode, int ndigits, int *decpt, int *sign, char **rve);
+char *ruby_hdtoa(double d, const char *xdigs, int ndigits, int *decpt, int *sign, char **rve);
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* util.c (export) */
+extern const signed char ruby_digit36_to_number_table[];
+extern const char ruby_hexdigits[];
+extern unsigned long ruby_scan_digits(const char *str, ssize_t len, int base, size_t *retlen, int *overflow);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_UTIL_H */