Lib.rs

Re-export important parts of bevy_ecs and bevy_app and make them more compatible with Azalea.

This is completely compatible with bevy_ecs, so it won't cause issues if you use plugins meant for Bevy.

Changes:

• Add TickPlugin, TickStage and AppTickExt (which adds app.add_tick_system and app.add_tick_system_set).
• Change the macros to use azalea/azalea_ecs instead of bevy/bevy_ecs
• Rename world::World to ecs::Ecs
• Re-export bevy_app in the [app] module.