move floor to C++20 (#1206)

move floor to C++20 (#1206)

* move floor to c++20

drop Visual Studio 2017.
use default member intializers for bit-fields.

* use more bit-field default initializers

* try Xcod 14.3.1 on macOS 13.

* echo clang version

which can be useful with homebrew fails_with

* use std::numbers

* finish eradication of M_PI.

* fix some whitespace