Support using C++17 std::byte for FromMemory functions

[walbourn]

When #497 was added to make the type of FromMemory operations use something other than void*, this turns out to result in code-breaks for clients using C++17's std::byte*.

This PR adds compat inlines to avoid requiring the client code add casts.

Minor tweak to CMake to avoid requiring all clients use the directxmath package externally.