unsafe_binder: implement v0 symbol mangling

[GrigorenkoPV]

Tracking issue: #130516
Fixes #154367

rustc_demangle also needs to be changed, but they are in a separate repo. I think I will put up a PR there once the suggested mangling is deemed OK.

[rustbot]

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[mati865]

I'm not familiar with the process for modifying the mangling spec, but I'd guess this requires some kind of agreement from the Compiler team Maintainers (not just the members) and rustc book update.

Hopefully we get lucky on reroll, otherwise I'll ask on Zulip.
@rustbot reroll