Make encode reusable in Phobos

[edi33416]

This PR adds two overloads to encode so those can be called from std.utf.encode (see here)

This PR depends on #2403

[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. Please verify that your PR follows this checklist:

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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#2404"

[RazvanN7]

@edi33416 Are you willing to take this to the finish line?