Use `__declspec(empty_bases)` on MSVC

[marzer]

Hiya,

For legacy/backcompat reasons MSVC doesn't automatically perform empty-base optimization, and requires you to opt-in using the empty_bases declspec.

This means the following code fails to compile on MSVC (but passes on GCC and Clang):

#include <strong_type/strong_type.hpp>

using myint = strong::type<int, struct my_int_>;

static_assert(sizeof(myint) == sizeof(int));

I have a patch to fix this locally, would you like it contributed as a PR? Edit: #61