Support reading from any generic Read + Seek device

In order to add partition support for Redox, ahcid and nvmed shall also provide block devices for the partitions. In order to do that, it makes much more sense for partitionlib to be more flexible, because it wouldn't make sense for those daemons to read from their own file descriptors. partitionlib currently only allows reading from Paths. I rewrote the MBR parser (the mbr crate didn't contain much at all) to work generically and patched the gpt crate.