Add forward declarations and typedefs for exposed structures

[davebayer]

This PR introduces vkFFT-fwd.h module that includes forward declarations and typedef of user exposed structures. The goal is to reduce the compilation times when including VkFFT in a header file. It does not contain any backend dependent code, thus is independent on the selected backend completely.

After this change, this behaviour is possible:

// Class.hpp
#include <memory>
#include <vkFFT-fwd.h>

class Class
{
public:
  // ...
private:
  struct VkFFTApplicationDeleter;

  // This works
  VkFFTApplication* mAppPtr{};

  // This also works
  std::unique_ptr<VkFFTApplication, VkFFTApplicationDeleter> mAppUniquePtr{};

  // Fails to compile as it would require require the backend dependent VkFFTApplication struct definition
  // VkFFTApplication mApp{};
};

// Class.cpp
#define VKFFT_BACKEND 1
#include <vkFFT.h>

struct Class::VkFFTApplicationDeleter
{
  void operator()(VkFFTApplication* app)
  {
    deleteVkFFT(app);
  }
};

// ...