Add low-overhead FixedAppender (internal for now)

[Oanaa28]

My PR revives the original PR (#8789).

[dlang-bot]

Thanks for your pull request and interest in making D better, @Oanaa28! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

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Testing this PR locally

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dub run digger -- build "master + phobos#10584"