deprecate features adopted into C++ (#1198)

* deprecated features adopted into C++

1) Mark the following GSL features as deprecated:
 - gsl::unique_ptr (always)
 - gsl::shared_ptr (always)
 - gsl::byte (since c++17)
 - gsl::joining_thread (never implemented)
2) Refactor existing deprecations to use the new GSL_DEPRECATED(msg) macro.
3) Create a section in the README for deprecated features in the
   standard.

* do not deprecate gsl::to_integer because we never claim to implement it.

* do not use gsl::byte if it is deprecated

include/gsl/pointers

@@ -18,6 +18,7 @@
 #define GSL_POINTERS_H
 
 #include "./assert" // for Ensures, Expects
+#include "./util" // for GSL_DEPRECATED
 
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
 #include <functional>   // for less, greater
@@ -62,8 +63,11 @@ namespace details
 //
 // GSL.owner: ownership pointers
 //
-using std::shared_ptr;
-using std::unique_ptr;
+template <typename... Ts>
+using shared_ptr GSL_DEPRECATED("Use std::shared_ptr instead") = std::shared_ptr<Ts...>;
+
+template <typename... Ts>
+using unique_ptr GSL_DEPRECATED("Use std::unique_ptr instead") = std::unique_ptr<Ts...>;
 
 //
 // owner