Impact of new C++20 math constants (<numbers>) on our code

C++20 added math constants to header <numbers> in the std::numbers namespace: http://eel.is/c++draft/numbers

The feature-test macro is __cpp_lib_math_constants.

There are the plain-named math constants (e, pi, sqrt2, etc), all of which are of type double, and there are the _v variable templates that hold template-argument-typed values of the corresponding constants.

There are two aspects to this:

1. we have our own FP type (qfloat16, and there's talk of a qfloat128)
   • we need to specialize the variable templates to work on qfloat16.
2. we may want to backport std::numbers as q20::numbers to use around our code