Consider cache pruning

[hadley]

Maybe using cachem?

[hadley]

Or just using the same implementation as cachem: https://github.com/r-lib/cachem/blob/main/R/cache-disk.R#L396-L467

Would require adding max_age, max_n and max_size parameters to req_cache(). Probably reasonable to default max_size to 1 Gb.

Probably not worth automatically clearing based on expires etc because that would require reading each file. But should check that we are deleting these files from the cache when we encounter them.