Use libloading instead of shared_library

[Rua]

Changelog (add to existing headers):

- **Breaking** Changes to `Instance` and Vulkan initialization:
  - `VulkanLibrary::get_instance_proc_addr` is now `unsafe`, and returns `ash::vk::PFN_vkVoidFunction`.

This replaces the shared_library crate with libloading. The original library was created by Tomaka, but has not been maintained since. The replacement crate is more up to date and much more widely used.

[AustinJ235]

Looks good!