[FEATURE] Add a simple string cache

[oliverklee]

This class then will later be used to replace the current caching
that is based on nested arrays.

Part of #144.

[JakeQZ]

Could the class implement ArrayAccess (instead of the set/get/has methods)? This would also mean fewer changes to the existing code which could continue to get, set and test the cached values using the [] operator and isset.

[oliverklee]

As far as I understand it, this would mostly remove the benefit of this class over using a plain array:

• typing the keys to strings
• typing the values to strings
• having more "speaking" method names.

So I'd prefer to now implement ArrayAccess for these reasons.

[JakeQZ]

• typing the keys to strings
• typing the values to strings

Yeah, I wondered about that, and can't think of or find a solution. Unlike C++, PHP does not have templates so it's not possible to implment ArrayAccess<string, string>.

[JakeQZ]

Looks generally good, and I can't think of any tests that are missing.

A few minor issues and questions.

[oliverklee]

@JakeQZ I immensely appreciate your detailed reviews! ❤️ On it.

[JakeQZ]

Lovely. (Sorry for the delay getting back to you, been away quite a but and also busy on other projects.)