Remove States::variants and remove enum-only restriction its derive

[ItsDoot]

Objective

The States::variants method was once used to construct OnExit and OnEnter schedules for every possible value of a given States type. Since the switch to lazily initialized schedules, we no longer need to track every possible value.

This also opens the door to States types that aren't enums.

Solution

• Remove the unused States::variants method and its associated type.
• Remove the enum-only restriction on derived States types.

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Changelog

• Removed States::variants and its associated type.
• Derived States can now be datatypes other than enums.

Migration Guide

• States::variants no longer exists. If you relied on this function, consider using a library that provides enum iterators.

[ItsDoot]

Something to consider is whether to remove the derive macro entirely, and blanket implement States.

[alice-i-cecile]

Something to consider is whether to remove the derive macro entirely, and blanket implement States.

I don't think we should, at least not in this PR. The explicitness of the trait has its upsides and this PR is not currently controversial.

[jnhyatt]

I agree we keep the derive macro, at least for now. I suppose there's no real reason anymore since States impl is empty.

[jnhyatt]

I'm going to link #9957 here because it's something we'll want to look into merging once this is in -- it adds the ability to match on state patterns in run conditions and transition schedules. Definitely something we want if we're allowing "anything can be a state"

[lee-orr]

I do want to add that I think having the derive is very useful - for a couple of reasons:

a) even with all the changes here and in my PR, states still need to be Eq and discreet in some way. Even if theres infinite* options, you have to be able to say "were in the same state we were in last time".
b) the convention across bevy is to have a derive for different categories of types that we support - components, resources, etc. removing that for states will feel arbitrary in a way, and make it harder to tell when something can or cannot be considered a state