Automagic GC

I feel that we need to think about garbage collection now. /home on drogon ran out of space, and fscacher has contributed -- e.g. in my case it was 4GB for `~/.cache/fscacher`.

- Ideally it should some kind of a LRU mechanism.
Unfortunately I think we cannot rely on atime for deciding which cached files to be removed since filesystem might be mounted with `relatime or noatime`, so we might need another way to decide on what is recent and what is not.

- another idea could be just to help user to decide by creating lead directories for those extra tags (versions) we add into function signature (but they could become too long too fast :-/). But we could then may be make decision based on that level -- the oldest created/modified ones to go. If it was modified recently - means that the particular version is still in use

Any ideas @jwodder ?