I want to standardize _all_ of Catch2 headers on the `.hpp` suffix.
- Overrides added - usages of push_back() replaced with emplace_back() - Loop variable made const-refernce - NULL replaced with nullptr - Names used in the declaration and definition unified - size() replaced with empty - Identical cases merged
Ideally they would not be used at all, but the main config lifetime is a weird mess right now and will require further refactoring.