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/isprintrune.c

@@ -0,0 +1,10 @@
+/* Automatically generated by mkrunetype.awk */
+#include "../utf.h"
+#include "runetype.h"
+
+int
+isprintrune(Rune r)
+{
+	return !iscntrlrune(r) && (r != 0x2028) && (r != 0x2029) &&
+	       (r < 0xFFF9) && (r > 0xFFFB);
+}