Modules define lists of objects, e.g. a `SpriteRenderer` module may define
```zig
sprites: mach.Objects(struct {
// ...
}),
```
Previously, the only way for another Mach module to 'attach data to a sprite object' or 'tag a sprite' would be to (ab)use the graph relations system, creating their own object and using parent/child relations to express that the sprite has some tag/data associated with it. For example:
```zig
// Game.zig
is_monster: mach.Object(struct{}), // empty object just to indicate 'some other object is a monster'
// ...
// Create a 'tag object'
const is_monster_tag_obj_id = game.is_monster.new(.{});
// Add the 'tag object' as a child of our sprite
sprite_renderer.sprites.addChild(my_sprite_id, is_monster_tag_obj_id);
// ...
```
This usage of the API was quite ugly/usage, and importantly eliminated your ability to use the parent/child relations for _other_ things where they are more appropriate. However, it did mean that you didn't have to go and fork+modify the `SpriteRenderer` module that you e.g. imported as a reusable package.
With this change, we add object _tags_ and _tags with values_. Any module can add their own tags or tags with values to any object, whether it is from their module or not. For example, the `is_monster` example above could now be written as:
```zig
// Game.zig
pub const mach_tags = .{ .is_monster };
// ...
try sprite_renderer.sprites.setTag(sprite_id, Game, .is_monster, null);
const is_monster: bool = sprite_renderer.sprites.hasTag(sprite_id, Game, .is_monster);
// is_monster == true!
// No longer a monster
try sprite_renderer.sprites.removeTag(sprite_id, Game, .is_monster);
```
This allows for effectively tagging objects as distinct kinds, states, etc. even though they aren't our object and we can't modify their `struct {}` type to include an `is_monster: bool` field of our own.
Internally, the implementation stores tags using a hashmap under the assumption that not all objects in a list will have a tag.
Tags with values work almost identically, the only difference is that the last parameter to `setTag` is set to another `mach.ObjectID` which points to whatever arbitrary data you'd like to attach to the object, and `getTag` returns it. For example:
```zig
// Game.zig
pub const mach_tags = .{
/// Whether a sprite is a monster
.is_monster,
/// Whether a sprite has a friendly sprite attached to it
.{ .friend, Sprite, .sprites },
};
// ...
try sprite_renderer.sprites.setTag(sprite_id, Game, .friend, friendly_sprite_id);
const has_friend: bool = sprite_renderer.sprites.hasTag(sprite_id, Game, .friend);
// has_friend == true!
// Get our friend
const friend_id: mach.ObjectID = sprite_renderer.sprites.getTag(sprite_id, Game, .friend);
// friend_id == friendly_sprite_id
// Delete our friend
try sprite_renderer.sprites.removeTag(sprite_id, Game, .friend);
```
Signed-off-by: Emi Gutekanst <emi@hexops.com>
