create 4.2.1 release

replace BYTE_TYPE with a solution based on namespaces (#1201)

- A macro with the very generic name `BYTE_TYPE` is likely to collide with existing code, so get rid of the macro.
- The new solution is to provide a non-deprecated `byte` in the namespace `gsl::impl`.
  - Users of GSL should use `gsl::byte`, which is still deprecated when mapped to a `std::std::byte`.
  - GSL types and functions need to use `gsl::impl::byte` so they do not trigger the deprecation warning.
- The `gsl::impl::byte` return type in an exported function is not nice, it might mislead users to use that type in their own declarations. But the `BYTE_TYPE` solution is not better in this respect.

Co-authored-by: Werner Henze <[email protected]>

GSL v4.1.0 (#1163)

Fixes: #1161

Update version strings in README and CMakeLists.txt.

Updated tag 4.0.0

Merge pull request #892 from JordanMaples/dev/jomaples/gsl3.1.0

Update project version to GSL 3.1.0

Merge pull request #870 from beinhaerter/suppress_span_warning

suppress code analysis warning

Merge pull request #870 from beinhaerter/suppress_span_warning

suppress code analysis warning

Merge pull request #861 from JordanMaples/dev/jomaples/lwg3320

implement LWG3320

Merge pull request #826 from JordanMaples/dev/jomaples/ctad_fix_v2

fix ctad warning in llvm

span constructor no address (#723)

* Changed &arr[0] to std::array<T, N>::data and std::address_of to protect against overloaded operator&.

* Removed the usage of `std::addressof` because it is a C++ 17 feature. Using decay for C arrays instead.

* Add unit tests for #662.

* Added c++17 test configurations for clang5.0 and clang6.0

* fixed CppCoreCheck pointer decay warning