* Important - Dropped support for WooCommerce 3.0.
* Important - Added support for WooCommerce 3.6+.
* Tweak - Removed old WooCommerce Helper dependencies.
* New: Stop saving "Part of" meta key. Use `add_filter( 'woocommerce_mnm_order_item_legacy_part_of_meta', '__return_true' );` to continue saving.
* Fix: Correct discount calculations when not discounting product from regular price.