Add unlink filesystem operation

For checkers that write multiple files to disk, the possibility that files can be hard linked are inconvenient, as this means that earlier file writes are overwritten by latter ones by accident.

I think it would be useful to add this in a new Posix module, which would correspond to the Standard ML basis library.

Start by locating the existing checker file-writing code and the proposed new Posix module, then compare the Standard ML basis library's unlink operation. Done means the Posix module exposes unlink so checkers can remove a filesystem name before writing files that may be hard linked.

Written by the indexing model from the issue text.