Fewer allocations to put smaller bitset to stream

[AlexGuteniev]

The value of 128 bytes is arbitrary, could be greater if we are brave.

I kept variable _Len as opposer to _Bits constant, as constant propagation apparently engages a very harmful optimization.

Didn't benchmark precisely output to stream, I think if falls under "definitely good" category; if not, I can add, but then will need to wait for #4817

[AlexGuteniev]

Can go furthr and convert to digit array by parts, and stream those parts, this way can avoid sllocations altogether.

But this would be more complex change

[StephanTLavavej]

I think this is plenty - large bitsets are increasingly uncommon, and anyone touching iostreams isn't going to have great perf in the first place.

[AlexGuteniev]

anyone touching iostreams isn't going to have great perf in the first place

Well, stringstream is not as that, but generally agree.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for letting the allocator take some vacation days! 🏖️ 🥳 😹