Fast unboxing of Box<T> value without unnecessary type checking

[AlexRadch]

Closes #822

PR Checklist

• [x] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [x] Based off latest main branch of toolkit
• [x] PR doesn't include merge commits (always rebase on top of our main, if needed)
• [x] Tested code with current supported SDKs
• [ ] New component
  • [ ] Pull Request has been submitted to the documentation repository instructions. Link:
  • [ ] Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• [x] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [x] Header has been added to all new source files (run build/UpdateHeaders.bat)
• [x] Contains NO breaking changes
• [x] Every new API (including internal ones) has full XML docs
• [ ] Code follows all style conventions

[Sergio0694]

The code being introduced relies on undefined behavior and is not safe. The whole type really is doing that too, which is why I plan on removing it altogether. There is no safe way of unboxing a boxed value bypassing type checks as of now, and relying on runtime internals is not the right approach. I also somehow missed marking this type as obsolete in the past 8.x release 😅