Use WeakMap

We are using in many cases some property directly on an object/function when we need to associate a value with that object. I think WeakMap support is good enough now that we can start using it and assume people will include a WeakMap polyfill for older browsers.

I think React is also using WeakMap.

## Todo
- [ ] use it in plugin-syntax-rule-value-function
- [x] use it in plugin-cache
- [x] update docs which refer required polyfils
- [x] search where else it could be used