Fix potential undoundness with Storage::as_slice and StorageMut::as_mut_slice

[sebcrozet]

Fix #828

• Add the safe methods: ContiguousStorage::as_slice and ContiguousStorage::as_mut_slice.
• The trait methods: Storage::as_slice, StorageMut::as_mut_slice have been renamed to Storage::as_slice_unchecked and StorageMut::as_mut_slice_unchecked and are marked as unsafe.
• The trait method Storage::is_contiguous is now marked as unsafe.

Not marking them as unsafe was invalid because they could result in invalid aliasing when calling as_mut_slice on two matrix slices obtained from the same matrix, but with overlapping (but non-contiguous) internal storage. For example, the following was allowed, but is actually unsound:

let (mut slice1, mut slice2) = matrix.rows_range_pair_mut(0, 1)
// Unsound because both mutable references point to the same element.
some_function(&mut slice1.data.as_mut_slice[1], &mut slice2.data.as_mut_slice[0]);