Use nested struct to provide constexpr constants

[jwnimmer-tri]

🦟 Bug fix

A portion of #269.

Summary

Use constexpr for simple static constants, to avoid the C++ global static construction and destruction order fiascos.

This is an alternative approach vs #283. Due to MSVC's apparent limitiations, the only way to provide inline static constants is via a helper struct (or namespace-scoped globals), so that the class Color exists as a complete type prior to declaring constants that use it.

Deprecate the const-only (non-constexpr) spelling of the constants.

At the moment, only Color has been refactored with this pattern while we discuss the implications.

In this version of the fix, we have an API+ABI break (with a deprecation transition), and thus will require users to refactor their code. In the #283 version of the fix, we remain API compatible (but still break ABI).

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[jwnimmer-tri]

I'm seeking input from project maintainers on whether this (#286) or else #283 is your preferred tactic to resolve the fiasco problem. Once I have that, I'll close the dispreferred PR, and update the preferred PR to cover all of the relevant types (Vector2, Vector3, etc.) that use the same pattern.

[mjcarroll]

Thanks for taking this on @jwnimmer-tri . I'm taking a look at both approaches and will let you know what I prefer, at least.

[jwnimmer-tri]

Closing in lieu of #283.