Add compression option for non-memory caches

[coolbutuseless]

Would it be possible to add a compression option for non-memory caches?

cache_filesystem, cache_gcs and cache_rds all use saveRDS() to serialise the object to disk, and the latter two then transfer over the network.

It would help if the user could tradeoff some CPU - by compressing the saved object - and hopefully get faster network transfers (and also stay within any cloud service size quotas).

I've submitted a PR with a proposed change to do this: #70