Split out from #94247
This adds the following methods to raw slices that already exist on regular slices
* `*mut [T]::is_empty`
* `*mut [T]::split_at_mut`
* `*mut [T]::split_at_unchecked`
These methods reduce the amount of unsafe code needed to migrate ChunksMut and related iterators
to raw slices (#94247)
Co-authored-by:: The 8472 <git@infinite-source.de>