Add par_rchunks* to ParallelSlice[Mut]

[cuviper]

[T] has these methods with no parallel in Rayon yet:

• pub fn rchunks(&self, chunk_size: usize) -> RChunks<T>
• pub fn rchunks_mut(&mut self, chunk_size: usize) -> RChunksMut<T>
• pub fn rchunks_exact(&self, chunk_size: usize) -> RChunksExact<T>
• pub fn rchunks_exact_mut(&mut self, chunk_size: usize) -> RChunksExactMut<T>

They're mostly like chunks(size).rev(), except they are aligned to the end of the slice, so any remainder/shortcoming is at the beginning of the slice.