Faas-cache-dict

Included
- Add `purge()` to empty all contents of cache dict

Todo:
- Discover better documents solution

This has not yet been tested in a prod environment, and as such should be considered beta software for now.

Included
- Fix `set_ttl()` should be able to make a value immortal by passing `None`
- Fix `is_expired()` no longer errors if the key has been purged, now returns True if missing
- Add ability to change the max (LRU) length of the cache once created, purging the LRU as required.
- Add extensive new documentation

Todo:
- Discover better documents solution

This has not yet been tested in a prod environment, and as such should be considered beta software for now.

Included
- Add FileBackedFaaSCache
- Add support for pickling faas cache objects
- Bump minimum supported version to Python 3.8
- Fix minor docs

Todo:
- Add full documentation

This has not yet been tested in a prod environment, and as such should be considered beta software for now.

Included
- Cleanup deprecated leftovers from objsize migration

Todo:
- Add full documentation

This has not yet been tested in a prod environment, and as such should be considered beta software for now.

Included
- Replaced primitive deep size algorithm with [objsize](https://pypi.org/project/objsize/)

Todo:
- Add full documentation

This has not yet been tested in a prod environment, and as such should be considered beta software for now.

Included
- Fixed "falsey" key causing badly reported item expiry. Thanks lokhman
- Fixed `get_deep_size` incorrectly passing params on deeply nested dicts.
- Fix handling of attempted nested deletion of already deleted items.
- Refactor constants to own file and general code cleanup.

Todo:
- Add full documentation

This has not yet been tested in a prod environment, and as such should be considered beta software for now.