Merge single layout descriptor set pools into StandardDescriptorSetAllocator

[marc0246]

SingleLayoutDescriptorSetPool, SingleLayoutVeriableDescriptorSetPool and StandardDescriptorSetAllocator were all doing almost the same thing, which resulted in there being 3 types of descriptor set allocations and descriptor sets as well, all in all I think it wasn't very clean which is what lead me to merge them into one.

While I was on it I also did some micro-optimizations, namely removing a HashMap<Arc<DescriptorSetLayout>, ...>> and replacing it with a sorted map using the IDs introduced in #2054, which resulted in the overhead of using StandardDescriptorSetAllocator over SingleLayoutDescriptorSetPool directly for example just barely being measurable any more. It's around 2 to 3 nanoseconds on my machine. Therefore, there is no incentive to keep the pools separate for those reasons either.

Also added some more utility functions for garbage collection to StandardDescriptorSetAllocator.

Changelog:

  ### Breaking changes
- - Renamed `SingleLayout[Variable]DescPool` to `SingleLayout[Variable]DescriptorSetPool` for consistency.
+ - Removed `SingleLayoutDescPool` and `SingleLayoutVariableDescPool`.