Add Instance::with_loading_function for better cross-platform handling

[tomaka]

Right now the Vulkan library is loaded in the loader module. Depending on the operating system the path to the library is different.

When trying to add support for MoltenVK, this became a problem because MoltenVK is a static library that must be linked at compile-time. PR #301 tweaks the loader module to have a totally different behavior on OSX.

However it would be great to not rely on any platform-specific behavior, in case the user wants to use a custom way of accessing Vulkan. Therefore I suggest adding Instance::with_loading_function:

impl Instance {
    pub unsafe fn with_loading_function(f: *const c_void, ... other params ...) -> Result<Instance, ...> {
        ...
    }
}

The function f passed to with_loading_function must be the implementation vkGetInstanceProcAddr, and will be used by vulkano to load everything.

The Instance::new function would be modified to use Instance::with_loading_function under the hood.

With this change Instance::new would try to work transparently on all major platforms, but the user could choose a custom implementation/behavior.

[tomaka]

Unfortunately what I didn't think about is that getting the list of layers and extensions also requires loading the function pointers.
Therefore we will need some sort of intermediate object I think.

Example:

let vk_library = unsafe { LoadedLibrary::from_function(...) };
let layers = vk_library.layers_supported_by_core();
let instance = Instance::with_library(vk_library).unwrap();