Refactor: eliminate unnecessary Option<BufferSize>

[kpreid]

Internally, BufferSlice and various commands take an Option<BufferSize> where None represents “the remainder of the buffer”. I believe, but am not certain, that this is a leftover from before wgpu::Buffers knew and exposed their own size (#2923), and the code may be simplified by making most or all of these internal Options non-optional. The public API would continue to accept Options for convenience; the size would be picked up from the Buffer object whenever it is not specified.

(I discovered this while working on BufferSlice and am making a note of it for later.)