* 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
