Use the now-stabilized pointer_bytes_offsets instead of what is technically UB and clean up the BufferContents derive macro expansion

[marc0246]

The old derive expansion assumed slice pointer layout, and although it is surely getting defined soon, it's still better to get rid of it now that it's possible because pointer_bytes_offsets has been stabilized for long enough, as the old expansion was UB. This also cleans up the expansion by moving more of the code into the library so that it's easier to catch bugs and to look at the expansion.

Changelog:

### Public dependency updates
- Rust version: 1.75.0