9th CRAN release

Another attempt to hotfix an issue with Cache and timestamps on low-precision file systems (such as ext3 and old Windows versions). This mainly concerns a few unit tests and no real-world applications.

Hotfix for some macOS related unit tests

Added experimental Cache R6 class for persistent, file system based caches

Maintenance release to fix some bad unit tests that broke because of the year change

Finishing touches before public announcement

Fleshed out R6 API and prepared integration with lgr

first CRAN release