Split up is*rune() and to*rune() functions into individual source files

This optimizes the binary size for each tool that uses these functions.
Previously, if a program just used one single function, maybe even a
one-liner, it would statically compile in all lookup-tables, bloating
the binary by up to 20K.
All these changes are derived from a local libutf where I do the
primary changes. So I hope that I can merge these things into libutf
sooner or later, as discussed on the ml.

libutf/runetype.h

@@ -0,0 +1,26 @@
+/* MIT/X Consortium Copyright (c) 2012 Connor Lane Smith <cls@lubutu.com>
+ *                            (c) 2015 Laslo Hunhold <dev@frign.de>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#define nelem(x)  (sizeof (x) / sizeof *(x))
+
+int rune1cmp(const void *, const void *);
+int rune2cmp(const void *, const void *);