fix for platform inconsistency in creation time

this introduces a new field to the filestat struct. since there's no syscall that retrieves the creation time of a file on linux, and the ctime field *does* refer to the creation time on windows, i've made a split between the two to identify the difference. where one is set, the other is 0 on the two platforms.
I don't think there's a "changed" field in the `FileStat` structure, does this compile?