This allows users to skip creation of `BufferSlice` if they have no use
for it, and brings `wgpu` closer to the WebGPU API without removing any
Rust convenience. New functions:
* `BufferSlice::slice()`
* `Buffer::map_async()`
* `Buffer::get_mapped_range()`
* `Buffer::get_mapped_range_mut()`
* `Buffer::get_mapped_range()`
