Make rt utf publicly available

[edi33416]

This is the first step in reusing some of druntime's utf.d implementations in Phobos.

Please see forum discussion.

[dlang-bot]

Thanks for your pull request and interest in making D better, @edi33416! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
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• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + druntime#2403"