assumeUnshared: convert shared lvalue to a non-shared one

[wilzbach]

Revival of https://github.com/dlang/druntime/pull/724 from the dead.

Changes from #724:

• rebased
• fix doc/style nits
• added changelog entry

This would be a great use case for an experimental stage or an marking a symbol as "unstable" for a few releases.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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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 run digger -- build "master + druntime#2156"

[jercaianu]

@wilzbach Is this still under work? I really need this Thanks!

[jacob-carlborg]

@wilzbach ping, what is the status of this?

[wilzbach]

@wilzbach ping, what is the status of this?

Needs approval from @andralex :/

[RazvanN7]

@andralex I have rebased and removed the immutable overload. Is this good to go now?

[RazvanN7]

@MoonlightSentinel I have added a note about assumeUnshareds transitivity.