Archive refactor

~~Depends on #279.~~

This is a general clean-up of the FS service emulation, including:
 - Move Kernel::Archive to Service::FS::Archive. (Archive doesn't belong in the kernel.)
 - Rename FileSys::{Archive, Directory, File} to ArchiveBackend, ArchiveDirectory and ArchiveFile, to prevent confusion with their Session frontends.
 - Simplify and improve lifetime management of Archive instances.
 - Implement 64-bit Archive handles. (Pretty crappy impl ATM, just sequential number. Maybe use randomized handles in the future.)
 - Remove the "Archive as a File" hack, which removes all the weird pesudo-duplicated interfaces from File that were present in Archive, the work-around where Archive handles were returned instead of files and the erroneous fact that Archive is a session. This is the main motivation for this PR.

Further work includes reconsidering if the front-end/back-end distinction is valuable anymore. (Probably not.) I would also like to move the entirety of the `file_sys/` directory and namespace over to inside the `service/fs/` directory, which is where I think the most logical place for them is. I'd like comments from the rest of the team on that.
It's required for the usage of make_unique in the later commits.