Partial libstdc++ Support

Hi all,

I enjoy working in C++ with avr-gcc. Since C++11, there have been a
number of low overhead features that can really make programming nicer.
Unfortunately, a fair number of these are part of the standard library,
and thus aren't available with avr-gcc.

Naturally, much of the C++ standard library isn't remotely suitable for
AVRs, and should not be supported. However, there are a fair amount of
features that I believe would work very nicely, and would be worthwhile
to support. For example, std::array, std::optional, <concepts>, and the
C++ counterparts to the C standard headers (e.g. <cstdint>).

It seems that the usual solution for those seeking this functionality is
to make their own implementation. While it works, I think it would be
nice to have it provided by avr-gcc, if possible. I'd prefer to keep the
boilerplate in the compiler rather than my projects.

Is there a reason beyond a lack of interest that these features are not
supported? I would be happy to work on this if so. Would selectively
picking out parts of libstdc++ be a reasonable approach?