Add std::span implementation

[messmerd]

This is a C++20 std::span implementation for C++17, plus a collection of compile-time unit tests.

This implementation of std::span does not include support for ranges (since we don't have ranges yet), but it does include the following extra features from later standards:

• C++23: Const iterators
• C++26: std::span over an initializer list P2447R6
• C++26: std::span::at() P2821R5

[Rossmaxx]

Sorry for being that guy but it would be better to wait till C++ 20 instead of rolling out our own implementation. Is there an urgent need to use span now?

[messmerd]

Is there an urgent need to use span now?

Yes, this is something I'm using in the pin connector PR. I've moved it to this PR to make reviewing the pin connector easier once it's ready for review.

We probably won't be upgrading to C++20 within the next several months, and std::span is such an important vocabulary type to have, so I think rolling our own implementation is worth it. It is nearly identical to std::span so it should be trivial to swap it out for std::span when we do eventually upgrade to C++20.

[Rossmaxx]

Ok then. I didn't thought of any upcoming improvements when I asked the question.

[messmerd]

I'm closing this PR, because it looks like upgrading to C++20 will be much easier than I was anticipating.