Feat: implement fromNKI? using c++

[ppotapov-aws]

Implement FromNKI? functionality defined in lean using c++
Notes:

• We don't have a tensor library. I will defer until later with tensor implementation
• The access definition doesn't match lean definition. I can follow up here or on separate PR

[graebm]

I stopped an 1/8 of the way in to this review. But here's what I have

[graebm]

utterly trivial: don't need struct in these lines

In .c files, you need to put struct everywhere you use custom types. But in C++ it's unnecessary.

[graebm]

If you just take the string by value, it's way easier for users (and just as efficient). They can call it like: Error("oh no") instead of Error(std::move(std::string("oh no"))). I see you discovered that and made a TraceHelper(std::string msg) helper function later to get the same result as what I'm suggesting here. You wouldn't need that helper anymore

The only advantage of the "r-value only" version you made here is that users are forced to call move() themselves and can't accidentally do a copy instead. But you can see the downside, as you had to make a helper function to work around the clunkiness.

Suggested change

	Error(std::string&& m) : msg(std::move(m)) {}
	Error(std::string m) : msg(std::move(m)) {}

[graebm]

Don't need this if you make change to Error constructor I suggested above

Suggested change

	Error TraceError(std::string msg) {
	return Error{std::move(msg)};
	}

[graebm]

Don't need it in this PR, but I'd love a proper Result class.
Or use c++ exceptions.

[graebm]

This is returning a copy of T. Because it takes result by reference, but return a T by value.

We might want it to be more like std::get for variant and std::expected::value(). Returning either a reference, or rvalue, to what's inside

template<typename T>
T& get_ok(const TraceResult<T>& result) {
    return std::get<T>(result);
}

template<typename T>
T&& get_ok(TraceResult<T>&& result) {
    return std::get<T>(std::move(result));
}