[emscripten] Convert element hiding container to C++

[trev]

Background

See #4122 for the rationale. For maximal performance, all operations involving filter text should stay within Emscripten - including elemHide container.

What to change

• Implement ElemHide namespace (alternatively: class with default instance, like in #5141) exposing methods clear, add and remove to manage list of filters.
• Methods getUnconditionalSelectors and getSelectorsForDomain should return a _ElemHide_SelectorList object which is internally a list of filter references. This object should expose a selectorCount property as well as selectorAt and filterKeyAt methods. This approach makes getUnconditionalFilterKeys method unnecessary.
• Method getFilterByKey should allow retrieving a filter by its key. So the key needs to have the following properties:
  • it should uniquely identify a filter
  • invalid keys should be rejected
  • conclusion from the key to the filter (or from multiple keys to the relation between the corresponding filters) shouldn't be possible
  • ideally, keys shouldn't require additional data storage but rather be calculated from existing data using a randomized secret value
• Methods getSelectors and getException are unused and shouldn't be reimplemented.

Notes

Implementing filter keys is optional given that these are currently only used by the legacy Firefox codebase.

[abpbot]

A commit referencing this issue has landed:
​Issue 5142 - Convert Element Hiding to C++