Improve DeviceLayout convenience

[marc0246]

This greatly improves ergonomics of working with DeviceLayout, which is important considering I would like DeviceLayout to be the one shared way of specifying a layout for all allocation, like buffers and suballocations thereof. DeviceLayout::repeat et al. taking NonZeroDeviceSize was a mistake as that adds way too much boilerplate for little benefit. The added constructors bring us with line with std's Layout such that users don't have to import and create a Layout just to create a DeviceLayout.

Changelog:

### Breaking changes
Changes to memory allocation:
- `DeviceLayout::repeat` and `BufferContentsLayout::layout_for_len` now take `DeviceSize` as argument.
- `DeviceLayout::{from_layout,into_layout}` return an `Option` now.

### Additions
- Added `DeviceLayout::{new_sized,new_unsized,for_value}` for improved ergonomics when (sub)allocating buffers.
- Added `DeviceAlignment::of_val`.