Add UnsafeDescriptorSetCreateInfo

[Rua]

Changelog:

- **Breaking** `UnsafeDescriptorPool` creation parameters are given using `UnsafeDescriptorPoolCreateInfo`.
- **Breaking** The `alloc` and `free` methods of `UnsafeDescriptorPool` are renamed to `allocate_descriptor_sets` and `free_descriptor_sets`.
- **Breaking** The `alloc` method of the `DescriptorPool` trait is renamed to `allocate`.
- **Breaking** `DescriptorSetLayout::descriptors_count` is renamed to `descriptor_counts`, and returns a hashmap instead of a separate type.

Not too complicated here either. I removed the DescriptorsCount type, because the same effect could be achieved more flexibly with a hashmap.

[AustinJ235]

Overall looks good, but I think we should remove the use of the fnv crate. FnvHashMap was used in a time where the std implementation was slower than desired. Now days rust is using hashbrown since version 1.36 and is more performant. On top of the fnv hasn't been updated in 2 years.