PSR-6: Cache

Pull request for a Caching PSR.  This summary will be updated with the current status periodically.

## Status
The current text can be found here: 
https://github.com/Crell/fig-standards/blob/Cache/proposed/cache.md
https://github.com/Crell/fig-standards/blob/Cache/proposed/cache-meta.md

Please discuss any proposed changes on the mailing list before filing a pull request against this branch.

## To participate
* Please *do not* discuss details of this PR as a PR!  I know it's weird, but that's the way FIG works these days. :-)
* To discuss a specific section, comment on the FIG mailing list in the thread specifically for that section.
* To request a specific change based on discussion on the mailing list, open a pull request against the Crell/Cache branch from your own fork.  Please do not open PRs that have not already been discussed at least briefly on the list.


What about adding `isDefined()` as a shortcut for `isHit() && get() !== null`?