Add CachedMap

[fregante]

I implemented and used this before, I don't remember where. I think it looks like:

const map = new CachedFunction('name', {updater: ()=>{throw 'bad usage'}});

map.set = map.applyOverride; // approximately
map.get = map.getCached;
delete map.getFresh

The use is for situations where you have several items to cache individually, but don't want to refactor your code to use CachedFunction instead. Example

This would let me drop the legacy API and release v7.

I think CachedFunction could extend CachedMap but the types might make this impossible.