Improved continuous integration testing

[amklinv-nnl]

Now supports Windows/MSVC as well as Linux/gcc and MacOS/clang. Also tests for both 17 and 20 standard compatibility.

[amklinv-nnl]

The C++17 build is broken. I'll submit a separate PR fixing it.

[mhoemmen]

Fix suggestions

proxy_reference.hpp

friend auto abs(const derived_type& x) {
  if constexpr (std::is_unsigned_v<value_type>) {
    return value_type(static_cast<const this_type&>(x));
  } else {
    return abs(value_type(static_cast<const this_type&>(x))); // causes build error, possibly because it fails to exclude `std::abs` via the `= delete` idiom
  }
}

This code probably predates abs-if-needed. We could try changing the implementation as follows.

friend auto abs(const derived_type& x) {
  return impl::abs_if_needed(value_type(static_cast<const this_type&>(x)));
}

[amklinv-nnl]

@mhoemmen I implemented the changes you suggested, and the build/tests are passing. Kindly consider accepting this PR at your convenience!

[mhoemmen]

Thanks for the fixes! : - )