[META] Taxonomy API improvements

There are lots of issues related to improving the Taxonomy API, and some overlap:

• #106015: [performance] DB caching for \Drupal\taxonomy\TermStorageController::loadTree() →
• #698918: Reduce memory usage from \Drupal\taxonomy\TermStorageInterface::loadTree() when parent is specified →
• #1207326: Refactor taxonomy hierarchy API for performance, consistency, and convenience →
• #251595: Add taxonomy_term_load_descendents() →
• #499144: taxonomy_get_tree() generates data almost, but not quite, useful in taxonomy_save_term() →
• #1175156: Tests for $term->parent in Term::save() →
• #2858753: Drupal should prevent taxonomy term heirarchy loops at the data model layer →

Closed

• #569326: Add hook_taxonomy_vocabulary_info() →
• 📌 Machine readable names for taxonomy terms Closed: outdated
• #2543726: Make $term->parent behave like any other entity reference field, to fix REST and Migrate support and de-customize its Views integration →