style: make conversion of version string to int public

* src/parse-gram.y (str_to_version): Rename as/move to...
* src/strversion.h, src/strversion.c (strversion_to_int): these new
files.

src/strversion.h

@@ -0,0 +1,28 @@
+/* Convert version string to int.
+
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of Bison, the GNU Compiler Compiler.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef STRVERSION_H_
+# define STRVERSION_H_
+
+/* Convert VERSION into an int (MAJOR * 10000 + MINOR * 100 + MICRO).
+   E.g., "3.7.4" => 30704, "3.8" => 30800.
+   Return -1 on errors. */
+int strversion_to_int (char const *version);
+
+#endif