Add header `<numbers>` or a subset thereof

[ckormanyos]

Add the C++20 header <numbers> or a subset thereof, which (st least for a partial implementation) is very simple.

[jwakely]

Why not just copy it from upstream?

[ckormanyos]

Why not just copy it from upstream?

Hi Jonathon (@jwakely) that is a good idea.

As mentioned in another post, I'd like to get re-qcquainted with the new work on freestanding and potentially get more involved, as time permits.