remove_cvref unavailable with c++17

[sethalves]

On Windows with Microsoft's clang:

clang version 11.0.0
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: c:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\Llvm\x64\bin

I get this:

./copper.h(1400,55): error: no member named 'remove_cvref_t' in namespace 'std'
COPPER_STATIC_ASSERT((std::is_same_v<std::remove_cvref_t, _detail::voidval_t>));
~~~~~^
./copper.h(49,82): note: expanded from macro 'COPPER_STATIC_ASSERT'
COPPER_GET_MACRO2(VA_ARGS, COPPER_STATIC_ASSERT2, COPPER_STATIC_ASSERT1)(VA_ARGS)
^~~~~~~~~~~
./copper.h(58,79): note: expanded from macro 'COPPER_STATIC_ASSERT1'
#define COPPER_STATIC_ASSERT1(condition) ::copper::_detail::static_assert_ifc()
^~~~~~~~~
./copper.h(1400,70): error: 'Rhs' does not refer to a value
COPPER_STATIC_ASSERT((std::is_same_v<std::remove_cvref_t, _detail::voidval_t>));
^
./copper.h(1394,20): note: declared here
template <typename Rhs, bool is_buffered, template <typename...> typename... Args>
^
etc

[atollk]

Fixed by #14