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/byte

@@ -17,6 +17,8 @@
 #ifndef GSL_BYTE_H
 #define GSL_BYTE_H
 
+#include "./util" // for GSL_DEPRECATED
+
 #include <type_traits>
 
 #ifdef _MSC_VER
@@ -72,6 +74,12 @@
 #define byte_may_alias
 #endif // defined __clang__ || defined __GNUC__
 
+#if GSL_USE_STD_BYTE
+#define BYTE_TYPE std::byte
+#else // !GSL_USE_STD_BYTE
+#define BYTE_TYPE byte
+#endif // GSL_USE_STD_BYTE
+
 #if GSL_USE_STD_BYTE
 #include <cstddef>
 #endif
@@ -80,7 +88,8 @@ namespace gsl
 {
 #if GSL_USE_STD_BYTE
 
-using std::byte;
+using byte GSL_DEPRECATED("Use std::byte instead.") = std::byte;
+
 using std::to_integer;
 
 #else // GSL_USE_STD_BYTE
@@ -155,23 +164,24 @@ constexpr IntegerType to_integer(byte b) noexcept
 
 #endif // GSL_USE_STD_BYTE
 
+
 template <typename T>
 // NOTE: need suppression since c++14 does not allow "return {t}"
 // GSL_SUPPRESS(type.4) // NO-FORMAT: attribute // TODO: suppression does not work
-constexpr byte to_byte(T t) noexcept
+constexpr BYTE_TYPE to_byte(T t) noexcept
 {
     static_assert(std::is_same<T, unsigned char>::value,
                   "gsl::to_byte(t) must be provided an unsigned char, otherwise data loss may occur. "
                   "If you are calling to_byte with an integer constant use: gsl::to_byte<t>() version.");
-    return byte(t);
+    return BYTE_TYPE(t);
 }
 
 template <int I>
-constexpr byte to_byte() noexcept
+constexpr BYTE_TYPE to_byte() noexcept
 {
     static_assert(I >= 0 && I <= 255,
                   "gsl::byte only has 8 bits of storage, values must be in range 0-255");
-    return static_cast<byte>(I);
+    return static_cast<BYTE_TYPE>(I);
 }
 
 } // namespace gsl