Unify FixedTime and Time (proof-of-concept)

[nakedible]

PULL REQUEST ONLY FOR DISCUSSION, NOT MERGE READY BY FAR.

Objective

Current FixedTime and Time have several problems. This pull aims to fix many of them at once.

• If there is a longer pause between app updates, time will jump forward a lot at once and fixed time will iterate on FixedUpdate for a large number of steps. If the pause is merely seconds, then this will just mean jerkiness and possible unexpected behaviour in gameplay. If the pause is hours/days as with OS suspend, the game will appear to freeze until it has caught up with real time.
• If calculating a fixed step takes longer than specified fixed step period, the game will enter a death spiral where rendering each frame takes longer and longer due to more and more fixed step updates being run per frame and the game appears to freeze.
• There is no way to see current fixed step elapsed time inside fixed steps. In order to track this, the game designer needs to add a custom system inside FixedUpdate that calculates elapsed or step count in a resource.
• Access to delta time inside fixed step is FixedStep::period rather than Time::delta. This, coupled with the issue that Time::elapsed isn't available at all for fixed steps, makes it that time requiring systems are either implemented to be run in FixedUpdate or Update, but rarely work in both.
• If you're fixing a specific issue, say "Fixes #X".

Solution

• Add a maximum delta time that can limits how much can be added to the virtual game clock by a single frame update. This also limits how many fixed update periods are run per frame.
• Make Time report "fixed step clock" time while inside FixedUpdate.

...

• Add maximum_delta to Time that limits how much the virtual game clock can be advanced with a single update. Does not affect raw clock.
• Move FixedTime::period to be Time::fixed_period and remove FixedTime structure entirely.
• Make Time::delta, Time::elapsed and friends report time according to fixed steps inside FixedUpdate.
• Implementation detail: Separate out Clock data structure to make it more obvious what the independent tracked values are and to reduce code duplication.

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Changelog

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Migration Guide

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• If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
• Simply adding new functionality is not a breaking change.
• Fixing behavior that was definitely a bug, rather than a questionable design choice is not a breaking change.

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[nakedible]

Closing in favor of #8964