Added template argument deduction for not_null (#689)

* Added template  argument deduction for not_null

This allows compilers with c++17 support to infer template
instantiation types when calling not_null constructor:

    int foo(not_null<const int*> x);

    int main()
    {
        int t = 0;
        not_null x{ &t };
        return foo(not_null{ &t });
    }

* replaced deduction guides by a simple constructor

* Updated tests

* fixed check for availability of std::byte

* testing c++1z on clang

* fixed cmakelists extra endif

* include cstddef header for clang and gcc in pointers

* fixed a typo

* fix missing nullptr_t type

* fixed typo in CMakeLists.tst

* change approach to c++17 testing, step one: revert cmake testing, update clang5.0 package

removed using latest c++ due to clang5.0 failing,
update clang5.0 travis config to use llvm-toolchain-trusty-5.0

* addressed comments

include/gsl/pointers

@@ -77,6 +77,12 @@ public:
         Expects(ptr_ != nullptr);
     }
 
+    template <typename = std::enable_if_t<!std::is_same<std::nullptr_t, T>::value>>
+    constexpr explicit not_null(T u) : ptr_(u)
+    {
+        Expects(ptr_ != nullptr);
+    }
+
     template <typename U, typename = std::enable_if_t<std::is_convertible<U, T>::value>>
     constexpr not_null(const not_null<U>& other) : not_null(other.get())
     {