Unify all buffer types

[marc0246]

This PR aims to unify the types CpuAccessibleBuffer, DeviceLocalBuffer, BufferSlice and CpuSubbuffer (formerly two types, CpuBufferPoolSubbuffer and CpuBufferPoolChunk) into one: Subbuffer.

What this solves is the need for boxed trait objects, which can be detrimental to performance and might make the API harder to understand for newcommers. Another thing is that with the functionality spread like it is, some might not even be aware of what they can do. For example BufferSlice, was until now not that visible. With everything being in once place, people can discover things more easily. Another issue was that when slicing/indexing a buffer, since all the APIs expect an Arc<dyn BufferAccess>, you have to box every BufferSlice, which includes slices of slices, and would lead to a chain of Arcs as long as the number of slicings. That's a bummer because ideally all that needs to change is an offset and size in the existing one.

Another benefit of having only one type for buffers is type complexity: the BufferAccess, TypedBufferAccess and BufferAccessObject traits are gone and so are all the generics and trait bounds associated with them. Also, BufferView no longer has a generic type parameter and the BufferViewAbstract trait is also gone. I think this makes the API more approachable.

This PR also addresses the long-standing issue of the (then) CpuBufferPool only working for download. The (now) SubbufferAllocator can allocate subbuffers for any purpose.

Fixes #867, fixes #962, fixes #1281, fixes #1733, fixes #2126.

Changelog:

  # Breaking changes
- Changes to CPU buffer allocation:
- - Replaced `CpuBufferPool` with `CpuBufferAllocator`, which is now marked `!Sync` and no longer has a `T` type parameter. The type parameter was moved to the methods, to allow one allocator to allocate as many types of buffers as needed.
+ Changes to buffer allocation:
+ - Replaced `CpuBufferPool` with `SubbufferAllocator`, which is now marked `!Sync` and no longer has a `T` type parameter. The type parameter was moved to the methods, to allow one allocator to allocate as many types of buffers as needed.

- - Merged `CpuBufferPoolChunk` and `CpuBufferPoolSubbuffer` into `CpuSubbuffer`.'
+ Changes to buffers:
+ - Merged `CpuAccessibleBuffer`, `DeviceLocalBuffer`, `BufferSlice`, `CpuBufferPoolChunk`, `CpuBufferPoolSubbuffer` into `Subbuffer`.
+ - Removed `BufferAccess`, `TypedBufferAccess`, `BufferAccessObject`, `BufferInner` and `BufferViewAbstract`.

  # Additions
  - Added `BufferAllocateInfo`.

[marc0246]

It's ready. I wish the Discord bot would pick up on this.