func StringGet(s string, index int) rune {
	if !lisp.IsMultibyteString(s) {
		return lisp.ArefString(s, index)  
	}
	return MbStringGet(s, index)
}
StringGet returns string nth byte.
Result is not truncated to "byte" type, rune is returned.
package rt
import (
	"emacs/lisp"
)