feat: narrow for non totally ordered types (microsoft#986)

…red types

Borrowing from microsoft/GSL#986 (thanks @JordanMaples).

Closes #309.

* Modernize CMake build scripts, add presets
* General cleanup and modernization
* Migrate to namespace `gsl_lite`

Add `gsl_FEATURE_GSL_COMPATIBILITY_MODE` for toggling the `namespace gsl` alias and the `Expects()` and `Ensures()` macros.

* Remove `Owner()` and `implicit` macros

* Simplify `finally()`, `on_return()`, and `on_error()` implementation

...by removing the barely usable pre-C++11 implementation, disallowing moves for C++17 and newer, and defining `uncaught_exceptions()` as returning `int`.

* Add checks and warnings for non-standard ABI, and `gsl_CONFIG_ACKNOWLEDGE_NONSTANDARD_ABI` switch for suppressing ABI warnings

* Add link-time ABI incompatibility detection for MSVC using `#pragma detect_mismatch()`

* Avoid defining type traits conditionally (except for `integral_constant<>`, which I'd rather not compromise)

* Add CppCoreCheck testing with `gsl_lite` namespace

* Include `<version>` header if available

We are already using version checks at various places, implicitly relying on the header having been included through other headers.

* For libc++, configure `_LIBCPP_HARDENING_MODE` rather than `_LIBCPP_DEBUG`

...since the former has superseded the latter.

* Implement `span<>` with static extent

We adapt the `span_iterator<>` implementation, most of the `span<>` implementation, and several unit tests from Microsoft GSL. The new implementation also features the special container unwrapping support for the Microsoft STL.

* Remove deprecated span constructors from reference, `unique_ptr<>`, `shared_ptr<>`

* Remove deprecated span member functions `at()` and `operator()` for indexing

* Remove deprecated span member functions `length()`, `length_bytes()`, `as_bytes()`, `as_writeable_bytes()`

* Deprecate unsafe member function `as_span<>()`

* Un-deprecate span construction from `array<T const, N>` (not sure why this was deprecated)

* Remove deprecated `make_span()` overloads

* Remove deprecated functions `as_writeable_bytes()`, `basic_string_span<>::operator()`

* Implement `narrow<>()` and `narrow_failfast<>()` for non totally ordered types

Borrowing from microsoft/GSL#986 (thanks @JordanMaples).

* Define uncaught-exceptions internals before using them

* Lower CMake version requirement to 3.16

...as currently required for our AppVeyor builds

* Fix `gsl_STATIC_ASSERT_()` for C++98

That is embarrassing. How did it go unnoticed?

* Remove pre-C++11 `owner<>`

* Expand `gsl_ELIDE_()` to a no-op to avoid empty statements

* Update readme, add documentation

* Add package manager example projects