TrackedStorage (flagged component storage)

[Aceeri]

Having a TrackedStorage that would add/remove flags depending on if the component was modified would be a good bonus to performance in a lot of cases. For instance, if you only want to recalculate something once the component is modified.

One problem is if the user uses a mutable storage and iterates instead of joining then it will flag all of the components. This could be solved by comparing the two results on mutable iterations.

[Aceeri]

Adding associated types to the UnprotectedStorage may make this easier and less painful to use.
e.g.

trait UnprotectedStorage<'a, T> {
    type Target: 'a;
    type TargetMut: 'a;
    ...
    unsafe fn get(&self, id: Index) -> Target;
    unsafe fn get(&mut self, id: Index) -> TargetMut;
    ...
}

This would allow for the storages to return components with a wrapper type for various purposes, in this case it would track if the component was modified through Deref/DerefMut.

Current issues with this means adding a lifetime to the UnprotectedStorage bubbles up:
UnprotectedStorage<'a, T> + Component<'a> + MaskedStorage<'a, T>

It isn't an issue until you reach the typemap of TypeId -> Storage, since the type id requires a 'static bound and we require a variable/non-'static bound.

Issue for non-'static TypeId: rust-lang/rust#41875

[torkleyy]

From Gitter:

We decided to have a FlaggedStorage and a ChangedStorage.
The first one just flags components which were iterated over mutably (or fetched using get_mut), while the latter one does an equality check.

[torkleyy]

As commented above, this issue has been split into two parts. FlaggedStorage was already implemented by #93. Closing this now and creating a new issue for the ChangedStorage.