LWG-3570 `basic_osyncstream::emit` should be an unformatted output function

[StephanTLavavej]

LWG-3570 basic_osyncstream::emit should be an unformatted output function

This was voted into the Working Paper in October 2021, see #2236.

[AlexGuteniev]

Isn't this vNext?

[StephanTLavavej]

I think we just need to construct a sentry object - I don't see any ABI concerns here. Perhaps I'm missing something (I always find it hard to understand iostreams).

[AlexGuteniev]

I'm not sure if new code which constructs a sentry object can link with old code that doesn't