Introduce LocalBuffer

[masaori335]

A temporal buffer is needed in some cases. With this LocalBuffer, when the requested buffer size is smaller than the threshold (1024 in default), we can avoid allocating memory on the heap. Which brings us performance improvements.

@ywkaras pointed out this could be an alternative of Arena on
#6495 (comment)

Co-authored-by: Walt Karas wkaras@verizonmedia.com

[masaori335]

As @ywkaras said, this 1024 comes from the default arena block size.

[ywkaras]

Is this redundant with #6536 ?

[masaori335]

I'd like to keep these commits and PRs separated. #6536 is the first use case, but this could be used anywhere.

[SolidWallOfCode]

Seems a reasonable starting point. I have been working on something similar, but a bit more general.

The first is the MemArena class in libswoc++. That's not quite the same - it's based on the internal Arena class and is more useful for repeated temporaries.

The other is SplitVector<T,N> which would have the same API as std::vector but have a static part of N instances of T. LocalBuffer<4096> becomes SplitVector<char, 4096>. This can avoid some casting.

We might also consider bringing back the MemSpan class which is designed to help with passing back buffer descriptions.

Overall, though this seems a reasonable step at this point, since my other projects are still not ready for production.

[masaori335]

Pushed new commit address @SolidWallOfCode's comment. Also, I made the type of buffer generic.

[randall]

[approve ci autest]